We previously used `AliasingMapping`s and `Inaccessible` to model
`UnsafeCell` agreement. This abuses the notion of a mapping since one
doesn't ever actually want to change the aliasing of a pointer (and
certainly not to `Inaccessible`) - really this was meant to model
pointer casts which should never be performed. In addition to being an
awkward fit, the presence of `Inaccessible` meant that code could not
assume that any `Aliasing` invariant permitted reading, and so we had to
add extra machinery to work around this.

Future commits will use a different, simpler model for denoting
`UnsafeCell` agreement or disagreement.

While we're here, make `Read` slightly more permissive, implemented for
`A: Aliasing, T: Immutable` rather than just `A: Reference, T:
Immutable`.

Makes progress on #1122, #1866

gherrit-pr-id: I1ac2ae177a235083e33b09fc848423220d3da042

This commit replaces the `TransparentWrapper` trait with a more generic,
bidirectional `TransmuteFrom<T>` trait. `U: TransmuteFrom<T>` indicates
that `T` and `U` have the same sizes and that `T` can be transmuted into
`U` given certain alignment and validity invariant mappings.

Makes progress on #1122

gherrit-pr-id: I7af68cdaa5ea47ab245a942d1cbd8bc5e6407223