gp2c extension for the FpX API

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

Dear PARI developers,

I have added a GP2C extension to generate code that use the FpX ABI with
GP2C.

Here an example:

fun(P:FpX,Q:FpX,p:int) = P*Q+Q^2

GP2C will generate

return FpX_add(FpX_mul(P, Q, p), FpX_sqr(Q, p), p);

Note that the prime number will always be named p, and the modulus T.

Types available are Fp, FpX, Fq, FqX but more can be added (Flx, etc.)

This is useful if you have complex formulae that you want to
convert to C.  The function Fq_polmodular_eval was written that way.

Cheers,
Bill.