Signature verification, fix keypair constructor, cleanup.

72: The number is arbitrary right now, which is why the assert exists. CryptoPP doesn't have a number for evaluating the max and I derived this from signing 300 H(H(...)) messages. I want these numbers expressed in the code, as this is a manual type conversion which is unique to this code. (Converting to/from compact sipa/secp256k1 and der signatures)