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/peer/mod.rs

@@ -191,7 +191,7 @@ fn test_new_from_message2() {
     let mut message = Message::new_with_data(data);
     message.set_pointer(8);
 
-    println!("code: {}", message.get_message_code_u32());
+    println!("code: {}", message.get_message_code());
 
     let peer = Peer::new_from_message(&mut message).unwrap();