Store factorization of minimal & characteristic poly

[fingolfin]

Many applications need a factorization of these polynomials. It wasteful to compute it and then discard it again.

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I started this back in August and don't want it to be forgotten. Not sure if this version already fully works, and there is no test, hence I am putting on a draft label.

Ideally a similar change would be applied to GAP's methods for computing min/char polynomials, but since the ones in cvec are faster and so I am always using them anyway, and since it was very, very easy to do so here, I did not feel motivated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.69%. Comparing base (313cb17) to head (9cd1e3f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   60.62%   60.69%   +0.06%     
==========================================
  Files           9        9              
  Lines        5729     5739      +10     
==========================================
+ Hits         3473     3483      +10     
  Misses       2256     2256              

Files with missing lines	Coverage Δ
gap/linalg.gi	60.84% <100.00%> (+0.45%)	⬆️

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