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<>

main/performance.cpp

@@ -247,8 +247,8 @@ Array Performance::get_custom_monitor_names() {
 	Array return_array;
 	return_array.resize(_monitor_map.size());
 	int index = 0;
-	for (OrderedHashMap<StringName, MonitorCall>::Element i = _monitor_map.front(); i; i = i.next()) {
-		return_array.set(index, i.key());
+	for (KeyValue<StringName, MonitorCall> i : _monitor_map) {
+		return_array.set(index, i.key);
 		index++;
 	}
 	return return_array;