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<>
Implemented by request of @neikeq to advance in the GDExtension version of Mono.
* If a Resource type is missing upon load, it will be remembered together with its data (Unless manually overriden).
* If a Node type is missing upon load, it will be also be remembered together with its data (unless deleted).
This feature makes working with GDExtension much easier, as it ensures that missing types no longer cause data loss.
