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/message/server/message_factory.rs

@@ -336,7 +336,7 @@ fn a_transfer_denial_carries_the_token_and_the_reason() {
     let message =
         MessageFactory::build_transfer_denial_message(555, "Cancelled");
     let mut decoded = Message::new_with_data(message.get_buffer());
-    assert_eq!(decoded.get_message_code_u32(), 41);
+    assert_eq!(decoded.get_message_code(), 41);
     decoded.set_pointer(8);
     assert_eq!(decoded.read_int32(), 555);
     assert!(!decoded.read_bool());