Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

core/method_bind.cpp

@@ -30,7 +30,7 @@
 
 // object.h needs to be the first include *before* method_bind.h
 // FIXME: Find out why and fix potential cyclical dependencies.
-#include "object.h"
+#include "core/object.h"
 
 #include "method_bind.h"