fix(lib): route on the full four-byte message code

Dispatch read one byte of the code, so a code with a non-zero high byte
was handled as its low byte. Message::get_message_code now reads the
whole little-endian uint32 and handlers register by u32. PeerInit and
PierceFirewall, the only frames with a one-byte code, get their own
accessor for the listener's first frame.

soulseek-rs-lib/src/actor/server_actor.rs

@@ -803,7 +803,7 @@ impl ServerActor {
                         message
                             .get_message_name(
                                 MessageType::Server,
-                                u32::from(message.get_message_code())
+                                message.get_message_code()
                             )
                             .map_err(|e| e.to_string())
                     );