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/os/memory.h

@@ -197,4 +197,12 @@ struct _GlobalNilClass {
 	static _GlobalNil _nil;
 };
 
+template <class T>
+class DefaultTypedAllocator {
+public:
+	template <class... Args>
+	_FORCE_INLINE_ T *new_allocation(const Args &&...p_args) { return memnew(T(p_args...)); }
+	_FORCE_INLINE_ void delete_allocation(T *p_allocation) { memdelete(p_allocation); }
+};
+
 #endif // MEMORY_H