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.

tests/factor/factor.pl

@@ -92,6 +92,9 @@ my @Tests =
      ['bug-with-128-bit-uintmax_t',
       '340282366920938463463374607431768211355',
       {OUT => '5 31 2195370109167344925570158757624311041'}],
+     ['lt-5000-times-128-bit',
+      '1701071552237771378853409662551409288273703',
+      {OUT => '4999 340282366920938463463374607431768211297'}],
      ['h-1', '-h 3000', {OUT => '2^3 3 5^3'}],
      ['h-2', '3000 --exponents', {OUT => '2^3 3 5^3'}],
     );