Bignum handling is dodgy

[petertodd]

Doesn't appear we handle bignums > 32bits correctly, and the code itself is uncommented, undocumented, and doesn't handle errors in any sane way.

[posita]

I might have misunderstood the purpose of this issue, but other than creating a dependency, is there a reason not to use pyOpenSSL pyca/cryptography as an interface to the various BN_* calls? (See my comment on #30.)

[petertodd]

@posita The BigNum handling doesn't need an external dependency, especially how that Bitcoin Core doesn't actually use it. What needs to be done is just a review/cleanup of the code itself; this issue is just to remind me I have something outstanding to fix!