CRACK MITIGATION SYSTEMS AND TECHNIQUES FOR WATER- CONTAINING HOUSINGS SUBJECT TO FREEZING TEMPERATURES REFERENCE TO RELATED APPLICATION [0001] This application claims the benefit of U.S. Provisional Patent Application No. 63/209,505, filed on June 11, 2021 and entitled CRACK MITIGATION SYSTEMS AND TECHNIQUES FOR WATER-CONTAINING HOUSINGS SUBJECT TO FREEZING TEMPERATURES, the content of which is hereby incorporated by reference in its entirety. FIELD OF THE INVENTION [0002] This invention relates to housings containing water subject to freezing and more particularly, although not necessarily exclusively, to systems and techniques for accommodating freezing water within housings of, e.g., heat exchangers and other equipment of swimming pools and spas. BACKGROUND OF THE INVENTION [0003] Water-containing vessels such as pools and spas sometimes contain heaters for imparting heat to water of the vessels. Such heaters may be of various types. One type, a heat pump including a heat exchanger, is described and illustrated in U.S. Patent No. 7,721,360 to Bernardi, et al., whose entire contents are incorporated herein by this reference. [0004] A basic principle of a heat exchanger is that heat may be transferred from one fluid to another. In pool heating systems, a first fluid often may flow within tubes or coils inside a housing. A second fluid – water of the pool – may be configured to flow inside the housing but outside the tubes or coils. Heat from the first fluid thus can be transferred to the pool water as it passes through the housing. [0005] Well known is that water expands as it freezes. Accordingly, challenges may arise when pool water remains inside a housing of a heat exchanger when the heater is inactive. In particular, if the pool water within the housing freezes and expands, it may subsume the available internal volume of the housing and cause the housing to crack or otherwise fracture or break. When the pool water later thaws, it then may leak from the housing through the fractures. Pools owners conventionally attempt to avoid this problem by “winterizing” their pool equipment, which involves draining pool water from equipment and enclosures. These actions are not always feasible, however, nor are they always accomplished by pool owners prior to a freeze. SUMMARY [0006] Embodiments covered by this patent are defined by the claims below, not this summary. This summary is a high-level overview of various embodiments and introduces some of the concepts that are further described in the Detailed Description section below. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification of this patent, any or all drawings, and each claim. [0007] According to certain embodiments, equipment of a swimming pool or spa may be configured to accommodate expanded volume of water within the equipment as the water freezes. [0008] According to various embodiments, a crack-mitigation system for equipment of a swimming pool or spa includes (i) a housing configured to receive water of the swimming pool or spa and (ii) means, contained at least partially within the housing, for accommodating expanded volume of the received water within the housing as the water freezes and thereby reducing the likelihood of the expanded volume causing the housing to crack. [0009] In some embodiments, the equipment is a heat exchanger. In various embodiments, the accommodating means includes foam. In certain embodiments, the heat exchanger includes a coil defining an interior region in which at least some of the foam is positioned. [0010] According to some embodiments, a method of circulating water within a swimming pool or spa includes passing the water through a heat exchanger having a housing containing, at least in part, means for accommodating expanded volume of the water within the housing as the water freezes. [0011] Various implementations described herein may include additional systems, methods, features, and advantages, which cannot necessarily be expressly disclosed herein but will be apparent to one of ordinary skill in the art upon examination of the following detailed description and accompanying drawings. It is intended that all such systems, methods, features, and advantages be included within the present disclosure and protected by the accompanying claims. BRIEF DESCRIPTION OF THE DRAWINGS [0012] The specification makes reference to the following appended figures, in which use of like reference numerals in different figures is intended to illustrate like or analogous components. [0013] FIG.1 illustrates a cut-away view of part of equipment for a swimming pool or spa. [0014] FIG.2 illustrates the equipment of FIG.1 with a crack mitigation system according to embodiments. [0015] FIG.3 illustrates the equipment of FIG.1 with a crack mitigation system according to embodiments. DESCRIPTION OF THE INVENTION [0016] Described herein are freeze-protection systems and techniques for mitigating, if not altogether avoiding, cracking of housings containing water subject to freezing. Included as part of the inventive concept may be a crack mitigation system with means for absorbing or otherwise accommodating within the housing the increased volume caused by expansion of water as it freezes. In some versions of the invention, the absorbing or accommodating means includes foam. Alternatively, the absorbing or accommodating means may include air and/or some other compressible material or substance (or combinations of materials or substances) as desired. Indeed, some other versions of the invention contemplate placing air in a sealed flexible enclosure within the housing, with the enclosure changing size and shape when compressed by expanding water volume. Various other benefits and advantages may be realized with the systems and methods provided herein, and the aforementioned advantages should not be considered limiting. [0017] FIG. 1 illustrates part of equipment 10 for a swimming pool or spa. In FIG. 1, the equipment 10 is a heat exchanger 11; however, in other embodiments, the equipment may be other equipment for a swimming pool or spa as desired that receives and/or contains water. As illustrated in FIG.1, the equipment 10 includes a housing 14 defining a housing area 12. Water from a pool or spa may flow into and through the housing area 12, and such water may enter and/or exit the housing 14 via a port 16 such as an inlet or outlet. The heat exchanger 11 as the equipment 10 further includes a tubular coil 18 within the housing area 12. The tubular coil 18 may be various suitable conduits for containing the above-described first fluid while separating the first fluid from the second fluid (e.g., pool water). The coil 18 defines an interior region 20. During operation of the heat exchanger 11, the coil 18 may contain the above-described first fluid capable of transferring heat to pool water passing within housing 14 externally of coil 18. The size and shape of the equipment 10 and components thereof should not be considered limiting on the disclosure. [0018] Referring to FIG. 2, in certain embodiments, the equipment 10 may include a crack mitigation system 22 within the housing 14 for mitigating, if not altogether avoiding, cracking of the housing 14 containing water subject to freezing. In various embodiments, the crack mitigation system 22 includes an absorber 24 within the housing area 12. In some embodiments, the absorber 24 may be sized and shaped so as to fit wholly or partly within the interior region 20 defined by the coil 18. Regardless of size, shape, and positioning within housing 14, the absorber 24 may absorb and/or accommodate the increase in volume of water within housing 14 as the water freezes. The absorber 24 may be various suitable materials, substances, and/or devices for absorbing and/or accommodating an expanded volume of the water (or second fluid) within the housing 14 as the water freezes to reduce the likelihood of the expanded volume causing the housing 14 to crack. In certain embodiments, the absorber 24 may be a compressible material and/or a flexible material capable of changing size and shape to accommodate the expanded volume. In various embodiments, the absorber 24 may be a material that is more compressible and/or flexible than a material used to form the housing 14. [0019] In the embodiment illustrated in FIG.2, the absorber 24 is a foam structure 26, and the foam structure 26 is sized and shaped so as to fit wholly or partly within the interior region 20 of the coil 18. In the embodiment illustrated, the foam structure 26 has a tapered profile such that a diameter of the foam structure 26 within the interior region 20 is greater than a diameter of the foam structure 26 within the housing area 12 but outside the interior region 20. Such a tapered profile may be employed to control the absorption and/or compressibility of the absorber 24 to have a particular profile and/or direction, such as encouraging compression into the interior region 20 as illustrated. The foam structure 26 may be constructed from various materials and have various structures as desired. As a non-limiting example, the foam structure 26 may be a polyurethane foam, such as an open cell or closed cell polyurethane foam; however, other types of materials may be used to form the foam structure 26 in other embodiments. [0020] FIG. 3 illustrates another example of the equipment 10 with the crack mitigation system. Compared to FIG. 2, in FIG. 3, the absorber 24 is a sealed flexible enclosure 28 with a compressible material (e.g., air or other materials) within the enclosure 28. In certain embodiments, the enclosure 28 optionally includes a port 32 such that air or other compressible materials can be selectively introduced into and/or removed from the enclosure 28, although it need not include such a port in other embodiments. The particular shape and size of the enclosure 28 should not be considered limiting. Similar to the foam structure 26, the flexible enclosure is positioned at least partially within the interior region 20 of the coil 18, although in need not be in other embodiments. As represented by the arrows 30 in FIG.3, the enclosure 28 may change size and/or shape when water within the housing 14 freezes, thereby accommodating the expanding water volume. [0021] In other embodiments, crack mitigation systems 22 may include other types of absorbers as desired for absorbing and/or accommodating the increase in volume of water within housing 14 as the water freezes. [0022] Exemplary concepts or combinations of features of the invention may include: A. Equipment of a swimming pool or spa configured to accommodate an expanded volume of water within the equipment as the water freezes. B. The equipment according to statement A, wherein the equipment is a heat exchanger comprising a housing and a coil within the housing. C. The equipment according to statement A or B, wherein the equipment comprises a crack mitigation system comprising an absorber within a housing of the equipment configured to accommodate the expanded volume of water within the equipment as the water freezes. D. The equipment according to any one of statements A-C, wherein the absorber comprises a foam structure. E. The equipment according to any one of statements A-D, wherein the equipment is a heat exchanger comprising a housing and a coil within the housing, wherein the coil defines an interior region, and wherein the foam structure is at least partially within the interior region. F. The equipment according to any one of statements A-E, wherein the absorber comprises a sealed enclosure filled with a compressible material. G. The equipment according to any one of statements A-F, wherein the absorber comprises a material that is more flexible than a material of the housing. H. A crack mitigation system for equipment of a swimming pool or spa comprising (i) a housing configured to receive water of the swimming pool or spa and (ii) accommodating means, contained at least partially within the housing, for accommodating or absorbing expanded volume of the received water within the housing as the water freezes and thereby reducing the likelihood of the expanded volume causing the housing to crack. I. The crack mitigation system according to statement H, wherein the equipment is a heat exchanger. J. The crack mitigation system according to statement H or I, wherein the accommodating means comprises a foam structure. K. The crack mitigation system according to any one of statements H-J, wherein the equipment comprises a coil within the housing, wherein the coil defines an interior region, and wherein the foam structure is at least partially positioned within the interior region. L. The crack mitigation system according to any one of statements H-K, wherein the foam structure comprises polyurethane foam. M. The crack mitigation system according to any one of statements H-L, wherein the accommodating means comprises a sealed enclosure with a compressible material. N. The crack mitigation system according to any one of statements H-M, wherein the compressible material comprises air, and wherein the enclosure is a flexible material. O. The crack mitigation system according to any one of statements H-N, wherein the accommodating means comprises an absorber having a flexibility greater than a flexibility of the housing. P. A method of circulating water within a swimming pool or spa comprising passing the water through equipment comprising a housing defining a housing region, the housing comprising accommodating means for accommodating expanded volume of the water within the housing as the water freezes. Q. The method according to statement P, wherein the equipment is a heat exchanger comprising a coil within the housing region, the coil comprising an interior region. R. The method according to statement P or Q, wherein the method further comprises positioning the accommodating means at least partially within the interior region of the coil. S. The method according to any one of statements P-R, wherein the accommodating means comprises a foam structure. T. The method according to any one of statements P-S, wherein the accommodating means comprises an absorber having a flexibility greater than a flexibility of the housing. U. The method according to any one of statements P-T, wherein the absorber comprises a foam structure or a sealed enclosure comprising a compressible material. [0023] These examples are not intended to be mutually exclusive, exhaustive, or restrictive in any way, and the invention is not limited to these example embodiments but rather encompasses all possible modifications and variations within the scope of any claims ultimately drafted and issued in connection with the invention (and their equivalents). For avoidance of doubt, any combination of features not physically impossible or expressly identified as non-combinable herein may be within the scope of the invention. Further, although applicant has described devices and techniques for use principally with equipment of a swimming pool or spa, persons skilled in the relevant field will recognize that the present invention conceivably could be employed in connection with filters, pumps, other objects, and in other manners. Finally, references to “pools” and “swimming pools” herein may also refer to spas or other water containing vessels used for recreation or therapy and for which water circulations or heating is needed or desired. [0024] The subject matter of embodiments is described herein with specificity to meet statutory requirements, but this description is not necessarily intended to limit the scope of the claims. The claimed subject matter may be embodied in other ways, may include different elements or steps, and may be used in conjunction with other existing or future technologies. This description should not be interpreted as implying any particular order or arrangement among or between various steps or elements except when the order of individual steps or arrangement of elements is explicitly described. Directional references such as “up,” “down,” “top,” “bottom,” “left,” “right,” “front,” and “back,” among others, are intended to refer to the orientation as illustrated and described in the figure (or figures) to which the components and directions are referencing. In the figures and the description, like numerals are intended to represent like elements. As used herein, the meaning of “a,” “an,” and “the” includes singular and plural references unless the context clearly dictates otherwise. [0025] The above-described aspects are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the present disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of the present disclosure, and all possible claims to individual aspects or combinations of elements or steps are intended to be supported by the present disclosure. Moreover, although specific terms are employed herein, as well as in the claims that follow, they are used only in a generic and descriptive sense, and not for the purposes of limiting the described embodiments, nor the claims that follow.