factor: speed up multiprecision Pollard’s rho

factor: speed up multiprecision Pollard’s rho

These changes are taken from a proposal by Torbjörn Granlund in:
https://lists.gnu.org/r/coreutils/2025-01/msg00000.html
On my x86-64 platform, they improve speed by more than 8× when
factoring 340282366920938463463374607431768211457.
* src/factor.c (mp_modadd, mp_modsub, mp_modadd_1, mp_mulredc):
New functions.
(MP_FACTOR_USING_POLLARD_RHO_N_MAX): New macro.
(mp_factor_using_pollard_rho): Act on mpn not mpz, and on
mp_limb_t not unsigned long int.  Reorder args.  All uses changed.