Update for new BIP340: XOnly is always even

[LLFourn]

Update to latest BIP340 that will now presumably be final. I completely removed the logic for SquareY. I think that there are systems like Grin that still use old BIP340 type signatures but I don't think it's worth keeping it in here for them at the moment.

In addition:

• I implemented the BIP340 domain separation scheme suggested here: https://github.com/sipa/bips/issues/207#issuecomment-673681901
• Created hex module for hex encoding/decoding since I needed it in tests and the logic was already mostly done in serialization code.