Add a new HashSet template

* Intended to replace RBSet in most cases.
* Optimized for iteration speed

scene/main/resource_preloader.cpp

@@ -29,7 +29,7 @@
 /*************************************************************************/
 
 #include "resource_preloader.h"
-
+#include "core/templates/rb_set.h"
 void ResourcePreloader::_set_resources(const Array &p_data) {
 	resources.clear();