An MSVC version of the 16->32-bit bignum optimisation, derived from part of

a patch by Lionel Fourquaux. Seems to be about a factor of four improvement
(see wishlist item for details).
I don't claim to understand this in detail, so I can't vouch for its
correctness, but it didn't fall over immediately. It also produces some
compiler warnings, unfortunately.

git-svn-id: svn://svn.tartarus.org/sgt/putty@6469 cda61777-01e9-0310-a592-d414129be87e