added a version of Brent Kung modular composition that takes a precomputed matrix as input
docu for new functions
use compose_mod_brent_kung_precomp_preinv in distinct degree factorization
ported compose_mod_brent_kung_precomp_preinv from nmod_poly to fmpz_mod_poly
use compose_mod_brent_kung_precomp_preinv in fmpz_mod_poly_factor_distinct_deg