DESCRIPTION COLD ROLUNG-MILL FOR WIRE AND RODS WITH IMPROVED ADJUSTMENT OFTHE DISTANCE BETWEEN CENTERS OF THE ROLLS Object The object of this invention is a cold rolling-mill for wire and rods with improved adjustment of the distance between centers of the rolls according to the preamble of the main claim. The installation including said rolling-mills is also part of this invention. Technical Field The technical field relates mainly but not exclusively to cold- rolling for wire and flats in the processes of reducing their section. Background Art The processes of cold reduction of wire by means of a die plate or idler rolls towed by capstans are known, as are the cold- rolling processes that use stands in which the rolls are motorized with spindles, like similar machines used in hot-rolling. Drawbacks of the Background Art As a rule, the movement of the rolls orthogonally to the wire requires the use of joints, spindles or other devices that allow the relative and parallel movement of the transmission axes. These solutions are complex, and have a limited performance. Problem to be Solved The aim of this invention is to produce machines and installations with very simple, inexpensive structures, with maximum reliability and performance. An additional aim is to improve the quality of the product. Solution to the Problem and Invention Statement The problem is solved according to the characteristics of the main claim. The sub-claims refer to preferred solutions and particularly advantageous sub-claims. Advantages In this way all the abovementioned problems are solved and all the desired results are obtained. Moreover, the machines and installation that adopt this technique improve the performance of the necessary reduction operations, by maintaining the wire rectilinear and using cold-rolling sequences in which compression and towing are carried out by rolls that do not use spindles. The compression sequences have been carefully conceived, and the line is shaped in such a way as to ensure, by means of its linearity, the entire series of advantages that will be illustrated below. Advantageously, eccentric approach in the eccentric adjusting system is allowed which, in addition to maintaining the main transmission axis fixed, parallel and axially rigid, allows a controlled rotation of the sleeves that bear the shafts on which the rolls are secured, in such a way a to avoid: on one side the incursion of water or dirt into the reducer, and on the other the escape of the grease and lubrication oil. Both rolls that tow the material to be processed are driven by at least one motor for each step or oval-round reduction sequence, the gears that transmit the motion to both rolling axes being permitted an appropriate clearance. Each reducer, that forms part of the rolling machine, consists of four gears placed in such a way that with a reduced clearance of said gears, there are relevant relative movements of the rolls, up to 20-30 times. That is to say, by fixing as an example a maximum gear clearance of 0.4 mm, a variation of the centre distances of no less than 10 mm is obtained. The rotation of the two sleeves, that concretize the variation of distance between centers, since they bear the rolling axes, occurs simultaneously by means of a screw equipped with half a right-hand thread, while the other half is left-hand and engages with the second sleeve. System with Overhanging Rolls In the rolling line, the overhanging fixing of the rolls allows easy interventions and the possibility of the rapid rotation/change of the worn out rolls. System with Central Rolls Supported from Both Sides This solution allows, by means of the prolongation of the eccentric sleeves, the support of the rolls on both sides, giving an extreme rigidity to the assembly. Rigidity is in fact necessary to allow strict tolerance and therefore also the quality of the end product. The change of the rolls in this case can be carried out rapidly by means of the easy extraction of the support pivot, an action that allows the removal of each roll orthogonally to the rolling axis. Accessories and auxiliary devices Each machine is completed by idler rolls placed orthogonally upstream of the rolling/towing rolls in order to produce the oval that prepares the material for compression into the bar, or is completed by differently shaped rolls in order to obtain any preparatory profile or simply for containing the wire and/or the camber in the case of production of flats. According to a variant, these rolls are also motorized in order to relieve the machine downstream of a part of the necessary towing force for pulling and producing the bar, the flat or any profile it has been decided to form in that step. Installation-Line of Cold Wire Production The wire/section cold production line consists of several machines structured to form several rolling steps; each step consists of at least one double orthogonal compression according to a sequence, idler rolls - motorized rolls or with both pairs motorized. With equal total reduction, this line allows a 20-25 % decrease in the number of steps in comparison with the number of steps necessary in a traditional machine. For example, a line that produces 1.8 mm wire in low-carbon steel from 5.5 mm wire rod, can be carried out with as few as 6 reduction steps rather than with the 8 that are normally necessary. The advantage of the system is also concretized by the fact that the always rectilinear wire, which aids the rapid insertion of the wire, avoids the well-known problems of sliding in the capstans, and moreover helps the production of rods, in addition to coils. Description of Preferred Embodiments The invention will be better understood with the aid of the following description of a non-limitative embodiment wherein: Fig.1A represents a rolling line for the production of wire or profile/s in six steps. Figs 1 ,2,3 represent a cold-rolling machine of the line, with the rolls supported on one side and the other, respectively: Fig.1 represents an elevated view on the vertical surface orthogonal to the feeding line of the wire (F) in section B-B compared to Fig.3; Fig.2 is an overview in section on the horizontal axial surface passing through the axis of the upper roll; Fig.3 is a view orthogonal to the axes of the rolls, in section on the vertical surface passing through the line A-A of Fig.2 and corresponding to the rotation system of the cams for the adjustment of the distance (INT) between the two axes of the rolls. Figs 3A, 3B, 3C represent schematically three adjusting stages in different positions of said distance between centers "INT", respectively minimum, intermediate and maximum distance. Figs 4,5,6 represent a cold-rolling machine of the line, with the rolls supported in an overhanging way, respectively: Fig.4 is an elevated view on the vertical surface orthogonal to the feeding line of the wire (F) in section passing through the axes of the superimposed rolls; Fig.5 is an overview in section on the horizontal axial surface passing through the axis of the upper roll; Fig.6 is a view orthogonal to the axes of the rolls, in section on the vertical surface passing through the line A-A of Fig.5 and corresponding to the rotation system of the cams for the adjustment of the distance (distance between centers=INT) between the two axes of the rolls. Figs.6A,6B,6C, represent schematically three adjusting stages in different positions of said distance between centers "INT", respectively minimum, intermediate and maximum distance. Detailed Description of the Figures As disclosed by the previous figures, SD indicates a machine with rolls supported on one side and the other "SD". This machine is the first of the rolling line. In the rolling line (Fig.1A), it can be seen that the five following machines are of the type with overhanging rolls (SA). Each rolling stand (SD, SA) includes in front of it a system with a pair of ovalizing press rolls, in order to allow the successive rolls of the rolling stand (SD1SA) to shape the wire into the definitive form of the step, in order to proceed then, progressively, to the reduction of the section until the definitive section as in the final machine (SA) to the right. Every stand includes a bearing structure indicated with (1) and a motor axis indicated with (2). The motor axis (2) transmits the motion to four known art gears (21 ,22,23,24) in pairs with an equal number of teeth, that form a quadrilateral gear train, in which the two terminations of the quadrilateral (23-24) coincide axially with the respective axes of the opposed rolls on the vertical surface (RR) which thus rotate at the same speed and in opposite directions. The inter-axle distance of the rolls, this distance between centers (INT) being adjusted by means of the rotation of an opening adjusting screw (VR) with identical and opposite threads which, in the same opposition, move an articulated leverism of cam rotation (TRE), whose cam is mounted on a sleeve (CE) coaxial to the axis of the rolls (RR) and which thus with its opposite rotation (3) increases or decreases the distance between centers (INT) between the rolls (RR). In this way said distance between centers (INT) is managed rapidly, precisely, safely and efficiently, as well as simply and micrometrically, adjusting very quickly and tuning up perfectly all the machines of the whole rolling line. Advantageously, the first machine of the rolling line, being with rolls (RR) supported by shoulders on both sides for a greater roll change efficiency, includes the rolls axis (Ax) extractable by means of a screwing head flange. Said axis includes a head cap (AX1), a cylindrical portion of central axis (AX2) and an internal end with a smaller diameter (AX3) that keys with the respective transmission gear for greater change efficiency and faster roll exchanges. In this way the maximum qualitative performance is obtained, as well as simplicity and functionality, reliability and rapidity. Advantageously as claimed: the adjusting system includes: i.- transmitting means with a single motor axis (1) with a four -geared transmission arranged as a quadrilateral (21,22,23,24) of which one motor and in pairs with identical gear teeth for each pair (21-22; 23-24) where one pair is rearward and intermediate (21-22) and the other is coaxial and spaced with opposite rolls (RR) with adjustable distance between their axes (INT); ii.- said axes of the rolls (RR) being mounted on identical and opposed cam sleeves (CE); iii.- said cam sleeves being associated to rotation means opposed in synchrony, consisting of: - coaxial screw means with right-left equal and opposed screw thread (VR), screwed into said opposite screw threads, into - articulated moving means (TRE) that move away or approach each other and at the same time cause the rotation of said cam sleeves (CE) in the same and opposite way, increasing or reducing in conformity the distance between centers (INT) of said rolls (RR). In the rolling line, in front of said pair of rolls (RR) a pair of orthogonal press rolls (RO) is provided. In this way the reduction and shaping of any section is facilitated. Like the first rolling-mill, at least one with rolls (RR) supported with stand shoulders on one side and the other (SD), followed by rolling- mills in which at least one includes said rolls (RR) supported in an overhanging way (SA). In this way, the greater reduction in section is better supported and performed, as the number and components of rolling stands can conveniently be combined and varied. Advantageously the mill with rolls (RR) supported with stand shoulders on one side and the other (SD) has a carrying axle for each extractable (AX) roll (RR) so that the respective roll (RR) can be extracted transversely to its rotation axis. In this way the rapid and efficient roll change is made much easier, like for the system with overhanging rolls. The number of the wires indicated in the Figures in contemporary working is two but it is evident that it can be different. Advantageously one or more of the rolling-mills of the line are rotated with a horizontal or vertical inclination. Advantageously, said inclination is 45°.