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<>

core/io/missing_resource.h

@@ -38,7 +38,7 @@
 
 class MissingResource : public Resource {
 	GDCLASS(MissingResource, Resource)
-	OrderedHashMap<StringName, Variant> properties;
+	HashMap<StringName, Variant> properties;
 
 	String original_class;
 	bool recording_properties = false;