1. using xonly_pubkeys in MuSig for input public keys and the combined
pk. For that to work we need to store whether the MuSig aggregated point
has an even y in the session, may need to negate each signers secret
key and may need to negate each signers public key in
musig_partial_sig_verify.
2. using a tagged hash for the message hash.
3. use !fe_is_odd in place of fe_is_quad_var
