This does not fail to compile with assertions about object pointers because it uses a Cell*. I'll add Cell* to my list of unallowed types.

It is difficult to do this for cell -- mostly because of unique id and the hashtables we use around the nursery -- so I think we're just going to have to wait for rekey to catch all of these.

It turns out it is also difficult to make anything else work with a bare Cell*. I think I'm going to try converting this to a variant first.

And we don't really want the CCW table to use UID anyway as there are just too many of them. Better to special case it for the nursery... assuming that that patch can ever land.