factor: refactor to for later performance speedup

factor: refactor to for later performance speedup

This does not affect performance much, but it should allow future
performance improvements.
* src/factor.c (factor_using_division): Two new args I and P,
which generalize this function.  All uses changed.
(mp_finish_up_in_single, factor_up): New functions, like the
non-*_up* versions but with two new args PRIME_IDX and PRIME.
They mostly just have the old body of the old non-*_up_ versions.
(mp_finish_in_single, factor): Rewrite in terms of the new functions.