added 2-1 division using Newton inversion with a precomputed inverse
added modular multiplication with a precomputed inverse
added modular powering with a precomputed inverse
added modular Brent-Kung composition with a precomputed inverse
changed distinct degree factorization so as to use precomputed inverse versions, less gcd's,
and less modular compositions
added benchmark test
added docu of mulmod_preinv
added docu for 2-1 division with precomputed inverse
added docu for powmod_ui_binexp_preinv
added docu of compose_mod_brent_kung_preinv