factor: use a more functional style

factor: use a more functional style

This is mostly to make the code a bit easier to read.
It shrinks the size of factor.o by 0.7% on my x86-64 platform
though it doesn’t affect CPU performance significantly.
* src/factor.c (mp_no_factors): Rename from mp_factor_init.
(mp_no_factors, mp_factor_using_division, mp_factor):
Return struct mp_factors rather than modifying one passed by reference.
All uses changed.