Add a new HashMap implementation

Adds a new, cleaned up, HashMap implementation.

* Uses Robin Hood Hashing (https://en.wikipedia.org/wiki/Hash_table#Robin_Hood_hashing).
* Keeps elements in a double linked list for simpler, ordered, iteration.
* Allows keeping iterators for later use in removal (Unlike Map<>, it does not do much
  for performance vs keeping the key, but helps replace old code).
* Uses a more modern C++ iterator API, deprecates the old one.
* Supports custom allocator (in case there is a wish to use a paged one).

This class aims to unify all the associative template usage and replace it by this one:
* Map<> (whereas key order does not matter, which is 99% of cases)
* HashMap<>
* OrderedHashMap<>
* OAHashMap<>

modules/gdscript/gdscript_parser.h

@@ -132,7 +132,7 @@ public:
 		ClassNode *class_type = nullptr;
 
 		MethodInfo method_info; // For callable/signals.
-		OrderedHashMap<StringName, int> enum_values; // For enums.
+		HashMap<StringName, int> enum_values; // For enums.
 
 		_FORCE_INLINE_ bool is_set() const { return kind != UNRESOLVED; }
 		_FORCE_INLINE_ bool has_no_type() const { return type_source == UNDETECTED; }