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

[Torbjorn Granlund]

I checked in some further mpz_powm_ui improvements.

mpz_powm_ui is still probably slower tnan mpz_powm for say b^19 mod N
when N is huge (> 2^100000).

Deeper hacking is needed to make things ideal.  See
<http://gmplib.org/devel/>.

-- 
Torbjörn