Add sorting, multiplication and gcd for fmpz_factor_t

[maelhos]

This PR aims at implementing some basic form of sorting to compute gcd and multiplication of factorization structures.
The end-goal is to use the fmpz_factor_t to do some bookkeeping, for example in the case of descent on elliptic curve.

Currently, the fmpz_factor_t structure does not have any invariants such has, being sorted which is definitely a trade-off to consider.

Also, I realized that implementing the sorting ended up not being as easy as expected because one needs to sort the mpz* but also applying the sorting permutation to the exponent vector. The qsort_r or qsort_s function exist for that purpose (otherwise sorting can't be done on-place if I'm not mistaken), but both of those function seem to have portability issues and requires #define _GNU_SOURCE. For now, I ended up re-coding a minimal implementation of quick-sort.

P.S. I remade the PR because previous squash desynchronized my fork.

[maelhos]

Maybe also having, _fmpz_factor_set which is used in test but not even in the source code, could also be a good idea. I'm working on some code to reduce and solve ternary quadratic forms that uses fmpz_factor_t pretty heavily to avoid refactoring.