Replace std::set with HashSet

[dementive]

This removes all of the std::set usage and replaces it with Godot's RBSet.

I'm making this a draft for now because it will not compile until https://github.com/godotengine/godot-cpp/pull/1841 gets merged, but once it does it will work fine.

[Ivorforce]

Note that HashSet is the recommended default 'set' type for Godot, the benefits of RBSet are not clearly established.

[dementive]

Note that HashSet is the recommended default 'set' type for Godot, the benefits of RBSet are not clearly established.

Just changed it to make it HashSet.

[dsnopek]

Looks like this one needs a rebase