rP24563 uses a GCHashTable to recognize objects.
I originally used MovableCellHasher, but that hit a VS17 bug. I then switched to a PointerHasher, and that seemed to work.
Unfortunately, it doesn't actually work, because it uses the object address and that can move. So I _do_ actually need a MovableCellHasher, as the docs explain.
I'll need to fix the VS17 bug.