Remove RES and REF typedefs in favor of spelled out Ref<>

These typedefs don't save much typing compared to the full `Ref<Resource>`
and `Ref<RefCounted>`, yet they sometimes introduce confusion among
new contributors.

core/io/resource.h

@@ -150,8 +150,6 @@ public:
 	~Resource();
 };
 
-typedef Ref<Resource> RES;
-
 class ResourceCache {
 	friend class Resource;
 	friend class ResourceLoader; //need the lock