The code we have for invalidating the shape teleporting optimization can be generalized into the initial Watchtower code. We can then build similar optimizations on top of it.

This factors out the code we have for invalidating shape teleporting and puts it
behind the initial Watchtower interface.

This adds a testing mechanism to call a JS callback for changes to watched objects.
We can use this to write tests for Watchtower hooks.

Depends on D136358

The compiler doesn't fold multiple ObjectFlag checks into one branch, because
relaxed-atomics are involved, so we have to do this explicitly.

Depends on D136359

This is a case we didn't handle correctly before the changes in bug 1750558.

Depends on D136360

Teleporting is handled under NativeObject::addProperty since bug 1738020, so the
case we're asserting against would no longer be a problem for us here.

Depends on D136361

At one point we might have had a subtle correctness bug here with shape teleporting
that this patch would have fixed, but nowadays it seems hard to trigger due to the way
shapes, prototypes and teleporting interact.

If not optimizing AddSlot in this case is a problem, we could support it with a
VMFunction, similar to what we do for (DOM) objects with an addProperty hook, or
we could allow a subset of Watchtower-watched objects based on the specific object
flags. For now this will hopefully be okay.

Depends on D136362

https://hg.mozilla.org/mozilla-central/rev/2384997f1201
https://hg.mozilla.org/mozilla-central/rev/bd2388ff62d7
https://hg.mozilla.org/mozilla-central/rev/bc30acd8634e
https://hg.mozilla.org/mozilla-central/rev/e49eeb1318e8
https://hg.mozilla.org/mozilla-central/rev/d0603ee735ba
https://hg.mozilla.org/mozilla-central/rev/6b946c5f5eb8