message: add AsRef to Signature so we can access the slice data
diff: @@ -101,3 +101,9 @@ impl<T, A: VerificationAlgorithm> Clone for Signature<T, A> {
  }
  }
  }
  +
  +impl<T, A: VerificationAlgorithm> AsRef<[u8]> for Signature<T, A> {
  + fn as_ref(&self) -> &[u8] {
  + self.signdata.as_ref()
  + }
  +}
  