mpz_powm_ui(...) is MUCH slower than mpz_powm(...)

[Zimmermann Paul]

I confirm mpz_powm_ui is much slower than mpz_powm:

tarte% ./benchmark-powm 100000000
pow2mod_ui ...
30011
46935641
1408585522051
Time for pow2mod_ui: 135.41 sec., 100000000 (1e+08) tests, 7.38e+05 tests/sec.

pow2mod_mpz ...
30011
46935641
1408585522051
Time for pow2mod_mpz: 56.67 sec., 100000000 (1e+08) tests, 1.76e+06 tests/sec.

This should be fixed.

Paul Zimmermann