Replace the BN_CTX implementation with my current work. I'm leaving the

little TODO list in there as well as the debugging code (only enabled if
BN_CTX_DEBUG is defined).

I'd appreciate as much review and testing as can be spared for this. I'll
commit some changes to other parts of the bignum code shortly to make
better use of this implementation (no more fixed size limitations). Note
also that under identical optimisations, I'm seeing a noticable speed
increase over openssl-0.9.7 - so any feedback to confirm/deny this on other
systems would also be most welcome.