Support size-shrinking value and reference transmutes

Support transmutations that allow shrinking the size of the value or referent.

This follows up on the work done in #1817.

## Progress

- [x] Support value transmutations (`transmute!`) that permit shrinking size
- #2581
- [ ] Support reference transmutations (`transmute_{ref,mut}!`) that permit shrinking size
- [ ] #2580
- [ ] #2487
- [ ] Support fallible value transmutations (`try_transmute!`) that permit shrinking size
- [ ] Support fallible reference transmutations (`try_transmute_{ref,mut}!`) that permit shrinking size