message: Use simpler / shorter Sha256::digest()
diff: @@ -206,7 +206,7 @@ mod tests {
  .unwrap());
  // Wrong message fails
  - let bad_message_hash = Sha256::new().chain([MSG, "\0"].concat()).finalize();
  + let bad_message_hash = Sha256::digest([MSG, "\0"].concat().as_bytes());
  assert!(!secp256k1_verify(
  &bad_message_hash,
  signature.as_bytes(),
  @@ -245,7 +245,7 @@ mod tests {
  let signature = hex::decode(sig).unwrap();
  // Explicit hash
  - let message_hash = Sha256::new().chain(&message).finalize();
  + let message_hash = Sha256::digest(&message);
  // secp256k1_verify works
  assert!(
  @@ -281,7 +281,7 @@ mod tests {
  let message = hex::decode(&encoded.message).unwrap();
  let hash = hex::decode(&encoded.message_hash).unwrap();
  - let message_hash = Sha256::new().chain(&message).finalize();
  + let message_hash = Sha256::digest(&message);
  assert_eq!(hash.as_slice(), message_hash.as_slice());
  let signature = hex::decode(&encoded.signature).unwrap();
  