cosi.verify was renamed to verify_combined, because it is pretty much
ed25519.verify, and the new name implies what it does in terms of the
CoSi scheme: verify a signature with already-combined public keys.
cosi.verify_m_of_n signature was simplified by not requiring the `n`
parameter, which is not important for verification. The updated function
was renamed to cosi.verify, because this is the standard CoSi
verification operation: given signature, digest, required number of
signatures, sigmask, and a list of public keys, verify that enough
signatures are indicated and that they sign the digest.
