Cross-compartment wrappers

[asajeffrey]

We need some way to allow JS-managed data in compartment C to point to data in compartment D, e.g. with a cross-compartment wrapper.

[asajeffrey]

Perhaps the way to do this is to provide a MaybeCCW<T> wrapper, which can be queried to either be a same-compartment T or a cross-compartment T::Transplanted where T: Transplantable<SOMEWHERE>. For example a MaybeCCW<Window<'a, C>> would either be a Window<'a, C> or a Window<'a, SOMEWHERE>.

If we only allow MaybeCCW<T> objects to be brain-transplanted, then their type isn't changed by transplanting.

Perhaps this should be MaybeXOW<T>, with three-options: a T, a cross-compartment same-origin T::Transplanted, or a cross-compartment cross-origin wrapper (in which case access to the T::Transplanted is unsafe???).