Make Frost signatures compatible with Bitcoin taproot / Schnorr signatures

[Lúcás Meier]

i.e. compatible with https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki

I think the best course of action here is to make a new wrapper, called frost-taproot or something like that, so that frost can still be useable for other situations. We'd want to make the internal challenge generation somewhat pluggable to define other variants.

Once we make protocols curve generic, then Frost will be completely pluggable for Ristretto or Ed signatures too.