Include reference implementation for X + Parity Keys

[JeremyRubin]

According to BIP-340, XOnly keys have 32 bytes and signatures have 64 bytes.

If X + Parity is used, signatures would be 65 bytes and Keys 33 bytes. Really, they just require one bit, but the minimum encoding unit is 8 bits, so there is 7 bits waste.

It would be useful to include a reference implementation here of X + Parity Schnorr signatures. A future upgrade to Bitcoin might choose this encoding. I'm not super familiar with this codebase, but perhaps if it is conceptually ACKd to include this in secp256k1, myself or others might do the work (v.s. if it's guaranteed to be rejected).