factor: factor insertion simplifications

factor: factor insertion simplifications

* src/factor.c (factor_insert_multiplicity):
Adjust to keep in sync with mp_factor_insert changes below,
by adding 1 to the index and using memmove to move.
(mp_factor_insert): Omit redundant call to mpz_cmp.
Prefer idx_t (always nonnegative) to ptrdiff_t,
by adding 1 to the indexes.
Prefer mpz_init_set to mpz_init+mpz_set.
Use memmove to move, rather than doing it by hand.