Merge rust-bitcoin#3357: Support impl AsRef<[u8]> in signed_msg_hash

70ccd6b signed_msg_hash takes impl AsRef<[u8]> (Liam Aharon)

Pull request description:

  Closes rust-bitcoin#3249

  Seems not required for Kixunil's immediate use case anymore, but figured I'd open a PR anyways.

ACKs for top commit:
  apoelstra:
    ACK 70ccd6b successfully ran local tests
  Kixunil:
    ACK 70ccd6b

Tree-SHA512: b15bafd1af0749cbac36652a4e4c9ab3f91b3980bb6c3cbc1c977d339810fc15075af5d4bc6b210de364bedc496ce3759eaf4539e40d215ab66a122ba69dd56a

bitcoin/src/sign_message.rs

@@ -205,12 +205,13 @@ mod message_signing {
 }
 
 /// Hash message for signature using Bitcoin's message signing format.
-pub fn signed_msg_hash(msg: &str) -> sha256d::Hash {
+pub fn signed_msg_hash(msg: impl AsRef<[u8]>) -> sha256d::Hash {
+    let msg_bytes = msg.as_ref();
     let mut engine = sha256d::Hash::engine();
     engine.input(BITCOIN_SIGNED_MSG_PREFIX);
-    let msg_len = encode::VarInt::from(msg.len());
+    let msg_len = encode::VarInt::from(msg_bytes.len());
     msg_len.consensus_encode(&mut engine).expect("engines don't error");
-    engine.input(msg.as_bytes());
+    engine.input(msg_bytes);
     sha256d::Hash::from_engine(engine)
 }