The taproot compatibility methods added to Ciphersuite were
very specific to taproot. I renamed them, and tidied up their
usage to remove unnecessary if/else branches. This should make
them applicable to any ciphersuite which needs to modify such
internal processes of the FROST algorithm.

This change should be a pure refactor with no logical changes.

I added the SigningTarget type, which encapsulates both the message
to be signed, and also the tapscript merkle root which the signature
should commit to. This allows a FROST group to dynamically select
what taproot tweak they would like to commit their signatures to,
and to optionally elect not to commit to any tweak at all if desired.

This method allows the group to export their tweaked
VerifyingKey, so 3rd parties can verify their signatures
without learning the taproot tweak.

BIP340 signatures are usually represented with x-only (even parity)
nonce points. As a step towards normalizing this for the frost-secp256k1-tr
crate, we should ensure all Signature struct instances always use the
effective nonce point, including the DKG proof-of-knowledge.

BIP340 signatures are supposed to be serialized as a 64-byte array:
32 bytes for the x-only nonce point 'R', and 32 bytes for the signature
component 's'. This commit customizes the frost-secp256k1-tr crate so
that signatures are serialized with x-only nonces, omitting the leading
parity byte.