factorff() for small prime

[Bill Allombert <allomber@math.u-bordeaux.fr>]

Hello PARI-dev,

I have commited some patches to make factorff(P,p,Q) faster for small
primes p. I have more patches to send still.

This mainly affect the special case where P had coefficients in Fp
(instead of Fq) but should also speed up the other case.

For that purpose I have added two new classes of functions:
FlxX_ and FlxqX_. They handle bivariate polynomials over a small
prime field (resp. univariate polynomial over a finite field)
and are stored as t_POL of t_VECSMALL.

Cheers,
Bill.