Creating detached signatures should work even if callers create a literal writer
Currently, this doesn't do the right thing:
let message = Message::new(&mut sink);
let message = Signer::new(message, signing_keypair)
.detached()
.build()?;
let mut message = LiteralWriter::new(message).build()?;
message.write_all(b"Make it so, number one!")?;
message.finalize()?;