Update to work with latest Godot 4.0

[rburing]

These are the minimal changes to make the module compile with the latest Godot 4.0.

Feedback and testing is welcome.

I noticed that Godot replaced the HashMap implementation, and godot_box2d at some points depends on the ordering of elements in a HashMap; I don't know if this requires any changes on the godot_box2d side.

[rburing]

I cherry-picked all the later changes from the 3.x branch, so after some testing this should be good to merge.