Added framework to sshbn.c to make it possible to vary the

underlying integer type forming the Bignum. Using this, arranged
that gcc/x86 uses 32-bit chunks rather than the guaranteed ANSI-
portable 16-bit chunks. This has gained another 30% on key exchanges
by my measurements, but I'm not yet convinced that it's all
perfectly robust - it seems to work fine for SSH1 and SSH2/RSA but
I haven't ensured that every bignum routine is actually being
tested, so it may yet show up problems in DSA or key generation.

git-svn-id: svn://svn.tartarus.org/sgt/putty@3135 cda61777-01e9-0310-a592-d414129be87e