Port some modular composition methods to generics

[fredrik-johansson]

Adds the following methods (plus underscore version) and replaces the counterparts in fmpz_mod_poly and fq_poly_templates with wrappers:

• gr_poly_mulmod
• gr_poly_mulmod_preinv
• gr_poly_compose_mod_horner
• gr_poly_compose_mod_brent_kung
• gr_poly_compose_mod
• gr_poly_compose_mod_horner_preinv
• gr_poly_compose_mod_brent_kung_preinv
• gr_poly_compose_mod_preinv
• _gr_poly_reduce_matrix_mod_poly
• gr_poly_precompute_matrix
• gr_poly_compose_mod_brent_kung_precomp_preinv

All have been ported from fq_poly_templates, with a few minor optimizations.

There are some interfaces I would like to change, and some of the methods need clearer specifications, in particular with respect to error handling. However, my goal for now is just to keep the fq_poly conventions to facilitate porting the polynomial factorization code to generics.

Note: this PR decreases the library size though here is a net + lines of code since some legacy functions and their tests can't be removed yet. Once the factorization code is ported to generics we will be able to remove more code.