message: remove the use of unwrap in the Signature's clone implementation
  it was safe to use unwrap here. However, it is better to remove it
  when the alternative is rather simple.
  relates to
diff: @@ -142,7 +142,11 @@ impl Signature {
  }
  }
  impl Clone for Signature {
  - fn clone(&self) -> Self { Self::from_slice(self.as_ref()).unwrap() }
  + fn clone(&self) -> Self {
  + let mut bytes = [0;SIGNATURE_SIZE];
  + bytes.copy_from_slice(&self.0);
  + Signature(bytes)
  + }
  }
  impl fmt::Debug for Signature {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  