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<>

scene/multiplayer/scene_replication_interface.cpp

@@ -49,9 +49,8 @@ void SceneReplicationInterface::make_default() {
 
 void SceneReplicationInterface::_free_remotes(int p_id) {
 	const HashMap<uint32_t, ObjectID> remotes = rep_state->peer_get_remotes(p_id);
-	const uint32_t *k = nullptr;
-	while ((k = remotes.next(k))) {
-		Node *node = rep_state->get_node(remotes.get(*k));
+	for (const KeyValue<uint32_t, ObjectID> &E : remotes) {
+		Node *node = rep_state->get_node(E.value);
 		ERR_CONTINUE(!node);
 		node->queue_delete();
 	}