factor: switch from mp to single when doable

factor: switch from mp to single when doable

This significantly improves performance when a number exceeds
2**(W_TYPE_SIZE - 1) and is the product of a prime less than
FIRST_OMITTED_PRIME and another prime less than 2**(W_TYPE_SIZE - 1).
On my platform, for example, it doubled the speed of factoring
4999 * (2**128 - 159).
* src/factor.c (mp_size, mp_finish_in_single): New functions.
(mp_factor_using_division, mp_factor_using_pollard_rho):
Finish using single precision when possible.
* tests/factor/factor.pl (lt-5000-times-128-bit): New test.