Modernize RWLock

- Based on C++14's `shared_time_mutex`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed
- Simpler for `NO_THREADS`

core/io/resource.h

@@ -149,16 +149,15 @@ typedef Ref<Resource> RES;
 class ResourceCache {
 	friend class Resource;
 	friend class ResourceLoader; //need the lock
-	static RWLock *lock;
+	static RWLock lock;
 	static HashMap<String, Resource *> resources;
 #ifdef TOOLS_ENABLED
 	static HashMap<String, HashMap<String, int>> resource_path_cache; // each tscn has a set of resource paths and IDs
-	static RWLock *path_cache_lock;
+	static RWLock path_cache_lock;
 #endif // TOOLS_ENABLED
 	friend void unregister_core_types();
 	static void clear();
 	friend void register_core_types();
-	static void setup();
 
 public:
 	static void reload_externals();