Various speed improvements to polynomial factorization and the GAP meataxe

[fingolfin]

While investigating the recog package together with Alice Niemeyer and Frank Lübeck last week, I came up with some tweaks to GAP that improved the recognition speed for some examples considerably.

First example: The following went from about 5.5 seconds to about 2.1 seconds on my laptop:

mat := IdentityMat( 300, GF(2) );;
mat{[1,2]}{[1,2]} := [[0,1],[1,0]]*Z(2);;
ConvertToMatrixRep(mat);;
M:=GModuleByMats([mat],GF(2));;
MTX.BasesCompositionSeries(M);; time;

Second example: The following went from about 78 milliseconds to 10ms. Over larger fields, it is not quite as dramatic, but still noticable (e.g. over GF(3), it went from 55ms to 13ms, and over GF(17) from 62ms to 39ms).

There are more optimization opportunities in the meataxe code. For example, it compute characteristic polynomials, then factors them. But the code for computing characteristic polynomials actually does so by first computing factors, then multiplying them... The obvious idea then is to provide a function which directly returns those factors (they won't be irreducible in general, but certainly still easier to factorize than the full char poly). Indeed, I noticed that the cvec package defines a method for that: FactorsOfCharacteristicPolynomial. Perhaps something like this should be added to GAP itself?

[markuspf]

These changes all look correct to me.

The only point I'd raise is that a potential speed gain from ConvertToMatrixRep should (maybe) be investigated in view of HPC-GAP (where ConvertToMatrixRep is deprecated).
This is of course not blocking this PR from being merged.

[hulpke]

Very Commendable. If ConvertToMatrixRep is depreciated (how do we this then?) there will be enough other problems that this one should not matter.

[frankluebeck]

This looks all fine to me.