This commit makes the following improvements:
- Removes the `Inaccessible` aliasing mode. This mode was not unsound,
  but it was unnecessary in practice.
- Replaces `Unknown` with `Unaligned` for alignment.
- Replaces `Unknown` with `Uninit` for validity.

Finally, with the exception of `transparent_wrapper_into_inner`, this
commit ensures that all `Ptr` methods which modify the type or validity
of a `Ptr` are sound.

Previously, we modeled validity as "knowledge" about the `Ptr`'s
referent (see #1866 for a more in-depth explanation). In particular, we
assumed that it was always sound to "forget" information about a `Ptr`'s
referent in the same way in which it is sound to "forget" that a `Ptr`
is validly-aligned, converting a `Ptr<T, (_, Aligned, _)>` to a
`Ptr<T, (_, Unaligned, _)>`.

The problem with this approach is that validity doesn't just specify
what bit patterns can be expected to be read from a `Ptr`'s referent, it
also specifies what bit patterns are permitted to be *written* to the
referent. Thus, "forgetting" about validity and expanding the set of
expected bit patterns also expands the set of bit patterns which can be
written.

Consider, for example, "forgetting" that a `Ptr<bool, (_, _, Valid)>` is
bit-valid, and downgrading it to a `Ptr<bool, (_, _, Initialized)>`. If
the aliasing is `Exclusive`, then the resulting `Ptr` would permit
writing arbitrary `u8`s to the referent, violating the bit validity of
the `bool`.

This commit moves us in the direction of a new model, in which changes
to the referent type or the validity of a `Ptr` must abide by the
following rules:
- If the resulting `Ptr` permits mutation of its referent (either via
  interior mutation or `Exclusive` aliasing), then the set of allowed
  bit patterns in the referent may not expand
- If the original `Ptr` permits mutation of its referent while the
  resulting `Ptr` is also live (i.e., via interior mutation on a
  `Shared` `Ptr`), then the set of allowed bit patterns in the referent
  may not shrink

This commit does not fix `transparent_wrapper_into_inner`, which will
require an overhaul or replacement of the `TransparentWrapper` trait.

Makes progress on #2226, #1866

Co-authored-by: Jack Wrenn <jswrenn@amazon.com>
gherrit-pr-id: I95d6c5cd23eb5ea6629cd6e4b99696913b1ded93