Use gp2c descriptions instead of prototypes

[jdemeyer]

Currently, the Cython code for wrappers of PARI functions is generated only using the "prototype". However, certain functions also have gp2c descriptions, which contains more fine-grained code. For example:

Function: Mod
Class: basic
Section: conversions
C-Name: gmodulo
Prototype: GG
Help: Mod(a,b): creates 'a modulo b'.
Description:
 (small, small):gen         gmodulss($1, $2)
 (small, gen):gen           gmodulsg($1, $2)
 (gen, gen):gen             gmodulo($1, $2)

The contains optimizations for Mod(x, y) if x or y are C longs instead of PARI objects.

It might not be worth using this, but at least we should think about it.

[videlec]

Might be useful for #52