Schnorr Signatures for secp256k1.

Defines Schnorr signatures for Bitcoin, a 64-byte signature type with stronger security proofs that lets several signers act as one.

Bitcoin first used a signature type called ECDSA. Schnorr signatures use the same elliptic curve and keys, but rest on a proven security basis, can't be altered by outsiders, are a fixed size and can be checked in batches. They also let several people sign together as one, which helps efficiency and privacy.