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<>
It used to always cycle all the peers when checking which one needed to
be notified, now it only does that when the target is multiple (i.e.
broadcast or exclusion).
Now uses two interfaces so it can be overridden in the future, and
core no longer depends on Node.
The interfaces are implements in scene/multiplayer.
Replaces root_node with root_path.
Remove all Node references from MultiplayerAPI.
