Heads up: some changes to value handle

Hi all,

This is a heads-up about a set of changes I'm about to check in which
(in net) change ValueHandles in the following way:

- The current WeakVH is renamed to WeakTrackingVH
- The current TrackingVH remains the same
- A new WeakVH is introduced that does not track values across RAUW,
but nulls itself out on value deletion

In other words, after the change value handles may have two
independent properties:

- Weak: nulls itself out on deletion
- Tracking: tracks values across RAUW events

and the set of handles are just the three non-empty unions of the two
properties above.

The motivation here is that for AliasSetTracker I needed a value
handle that did not track RAUW, but was still safe to access after
value deletion.

The changes in question are D32270, D32268, D32267 and D32266.

Thanks,
-- Sanjoy