Encapsulate message wire encoding into a module

Lightning messages are identified by a 2-byte type when encoded on the
wire. Rather than expecting callers to know message types when sending
messages to peers, have each message implement a trait defining the
message type. Provide an interface for reading and writing messages
as well as a Message enum for matching the decoded message, including
unknown messages.

Author: jkczyz

lightning/src/ln/mod.rs

@@ -24,6 +24,7 @@ pub(crate) mod peer_channel_encryptor;
 
 mod channel;
 mod onion_utils;
+mod wire;
 
 #[cfg(test)]
 #[macro_use] mod functional_test_utils;