Implement the Montgomery technique for speeding up modular

exponentiation by replacing the modulo operation by a cleverly chosen
multiplication. This was not worth doing in the previous state of the
code (because my multiply was about as slow as my modulo), but now
that multiplication has been sped up by the Karatsuba optimisation,
Montgomery becomes worthwhile.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9094 cda61777-01e9-0310-a592-d414129be87e