Nearly forgot. Reinstate the original unoptimised modpow, as a

Nearly forgot. Reinstate the original unoptimised modpow, as a
fallback for when Montgomery is inapplicable.

(I may also at some point switch to using it for small exponents, if
speed testing should reveal that there's a noticeable threshold beyond
which preparing the Montgomery setup is uneconomical.)

[originally from svn r9099]