LibIPC+IPCCompiler: Add IPC::Decoder, let classes decode themselves

This shaves ~5 seconds off of a full build, not too bad. Also it just
seems nicer to push this logic out to classes. It could be better but
it's a start. :^)

Libraries/LibGfx/Rect.h

@@ -334,3 +334,7 @@ inline void Point::constrain(const Rect& rect)
 const LogStream& operator<<(const LogStream&, const Rect&);
 
 }
+
+namespace IPC {
+bool decode(BufferStream&, Gfx::Rect&);
+}