This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
When a Reference managed instance is garbage collected and its finalizer is called, it could happen that the native instance is referenced once again before the finalizer can unreference and memdelete it. The workaround is to create a new managed instance when this happens (at least for now).
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
