This simplifies a tiny bit our bindings in some places, and complicates
it in others, but over all I think it's better.
It requires a bit more manual code in the rust side to drop and cast the
relevant pointers (which was done implicitly before), but it's a lot
less magic than it used to be, and that's all autogenerated so consumers
don't need to care about it.
The set up is still not ideal. I don't like that we rely on destructors
running in both sides of the FFI boundary, but that's for another day.
This is the last usage of RawOffsetArc, so remove that. We now support
proper Arc<> in structs (GridTemplateAreas uses it), so I don't think
we'll need it any time soon.
Differential Revision: https://phabricator.services.mozilla.com/D177905
2319f4f573