message: add placeholder verify_multi_signature
diff: @@ -95,6 +95,20 @@ where
  signature.clone().coerce().verify(public_key, &bytes)
  }
  +pub fn verify_multi_signature<T, A>(
  + signature: &crypto::Signature<T, A>,
  + public_key: &[crypto::PublicKey<A>],
  + data: &T,
  +) -> crypto::Verification
  +where
  + A: VerificationAlgorithm,
  + T: property::Serialize,
  +{
  + assert!(public_key.len() > 0);
  + let bytes = data.serialize_as_vec().unwrap();
  + signature.clone().coerce().verify(&public_key[0], &bytes)
  +}
  +
  /// A serializable type T with a signature.
  pub struct Signed<T, A: SigningAlgorithm> {
  pub data: T,
  