Update TransparentWrapper asserts

[zachs18]

1. Fixed the {wrap/peel}_slice methods asserts: previously they were comparing the layouts of *const Inner and *const Self, which is are always the same when they're both Sized; now they check that Inner and Self have the same layout.
   • This could cause panics for existing users with incorrect TransparentWrapper implementations.
2. Added asserts to wrap and peel.
   • This could cause panics for existing users with incorrect TransparentWrapper implementations.
3. Added comments explaining why we compare size of *const Self in {wrap/peel}_{ref/mut/rc/arc/box}.
4. Moved some of the asserts and safety comments before the unsafe blocks (no semantic change, I just prefer it).

(I was going to also change the assert!s to debug_assert!s, but since the conditions are all const-foldable anyway, the only affect that would have would be hiding UB in release mode, with no benefit for correct implementations, so I left it using assert!)