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<>
* Most resource types now have unique identifiers.
* Applies to text, binary and imported resources.
* File formats reference both by text and UID (when available). UID always has priority.
* Resource UIDs are 64 bits for better compatibility with the engine.
* Can be represented and used textually, example `uuid://dapwmgsmnl28u`.
* A special binary cache file is used and exported, containing the mappings.
Example of how it looks:
```GDScript
[gd_scene load_steps=2 format=3 uid="uid://dw86wq31afig2"]
[ext_resource type="PackedScene" uid="uid://bt36ojelx8q6c" path="res://subscene.scn" id="1_t56hs"]
```
GDScript, shaders and other special resource files can't currently provide UIDs, but this should be doable with special keywords on the files.
This will be reserved for future PRs.
