factor: avoid an mpz init+clear

factor: avoid an mpz init+clear

* src/factor.c (mp_factor_insert_ui): Rename fom
mp_factor_insert_ui, and change arg type from unsigned long int to
mp_limb_t.  All uses changed.  This avoids creating and freeing a
small mpz_t.
(mp_factor_using_division): Add a static assert requiring that
that mp_limb_t be wide enough, which it should be (and is in
standard GMP).