Model shrinking and overwriting transmutes in `pointer::transmute`

- `TryTransmuteFromPtr` only needs that the set of `DV`-valid referents of `dst` is a subset of the set of `SV`-valid referents of `src`. Note that this is slightly different than saying that the set of `DV`-valid values of `Dst` is a subset of the set of `SV`-valid values of `Src`. In particular:
- The former statement takes into account that we only care about a particular *size* of `Src` and `Dst` – we don't need to reason about *other* sizes (other than the size of *this particular referent*)
- The former statement is only *useful* when we're talking about size-preserving transmutes. If we're talking about size-shrinking, then this is insufficient.

Plan:
- [x] Step 1: Nail down current semantics, which don't permit shrinking transmutes
- Update `TryTransmuteFromPtr` with the requirement that "set of `DV`-valid referents of `dst` is a subset of the set of `SV`-valid referents of `src`"
- Update the proof on the impl of `TryTransmuteFromPtr` which does *permits* reverse transmutation – update it so that it relies on `SizeEq::CastFrom::project` being size-preserving.
- This is done: #2911
- [x] Step 2: Remove `TryTransmuteFromPtr: SizeEq` super-trait bound and permit *any* size-preserving transmute
- This is done: #2920
- [ ] Step 3: Permit shrinking transmutes
- Allow `SizeEq::CastFrom` to perform shrinking transmutes
- Update `TryTransmuteFromPtr` to handle *both* non-shrinking and shrinking transmutes (including the cases of *immutable* shrinking and *mutable* shrinking (which permits overwriting/tearing))
- Update the proof on the impl of `TryTransmuteFromPtr` which *does* permit reverse transmutation – add a `SizeEq::CastFrom: CastExact` bound so that it does *not* support shrinking
- The impl of `TryTransmuteFromPtr` which bans reverse transmutation (for shared `Immutable` pointers) now naturally supports shrinking
- [ ] Step 4: Support overwriting transmutes