Document the semantics of `Ptr` validity invariants on the `Validity`
trait.

Makes progress on #2226, #1866

gherrit-pr-id: Ie66db9044be1dc310a6b7280a73652a357878376

This commit removes the `TransparentWrapper` trait and the
`Ptr::transparent_wrapper_into_inner` method. It replaces them with a
new family of transmutation traits which encode more generic
transmutation (from any `T` to any `U`) and a set of `Ptr` methods which
use those traits to bound transmutation operations.

In particular:
- `Dst: TransmuteFrom<Src>` denotes that a by-value transmutation is
  sound
- `Dst: TryTransmuteFromPtr<Src>` denotes that a transmutation is sound
  so long as it can be guaranteed that the source is bit-valid for the
  destination; this is used by e.g. `Ptr::try_into_valid`, which
  performs runtime validation of bit validity
- `Dst: TransmuteFromPtr<Src>` is equivalent to `TransmuteFrom<Src> +
  TryTransmuteFromPtr<Src>`

Some type arguments are omitted in this summary. In particular, all
three traits also take validity invariant parameters for both the source
and destination types. Also, the `[Try]TransmuteFromPtr` traits take an
aliasing parameter.

In order to support these traits, we introduce a generalization of
`Read` known as `MutationCompatible`. `T: MutationCompatible<U, A>`
denotes that *either* `T: Read<A>` and `U: Read<A>` *or* `T` and `U`
have the same interior mutation semantics (formally, it is sound for
`&T` and `&U` to reference the same referent - safe code operating on
these references cannot cause undefined behavior). This is a refinement
of the "`UnsafeCell` agreement" concept that we have used before, but it
supports types which store but don't actually use `UnsafeCell`s. For
example, given a hypothetical `ReadOnly<T>`, the following bound holds:

  usize: MutationCompatible<ReadOnly<AtomicUsize>, Exclusive>

This commit also takes a different approach from the one originally
envisioned in #1945. In particular, it turns out that we don't need a
full type-level mapping concept. Instead, we need a *predicate* over
transitions to determine which ones are valid (e.g., it is valid to go
from a `Valid` `MaybeUninit<T>` to an `Uninit` `MaybeUninit<T>`). By
contrast, the invariant mapping concept suggests that each source
validity has *exactly one* destination validity.

This commit makes progress on #1940 by supporting unsized
transmutations, but we don't yet support size shrinking or expanding
transmutations.

This commit obsoletes #1359, as that issue was predicated upon the
existence of `TransparentWrapper`, which this commit removes.

This commit closes #1226, which suggests supporting `UnsafeCell`
agreement.

Closes #1945
Closes #1359
Closes #2226
Closes #1226
Closes #1866
Makes progress on #1359


gherrit-pr-id: Iad14813bc6d933312bc8d7a1ddcf1aafc7126938

Co-authored-by: Jack Wrenn <jswrenn@amazon.com>

Makes progress on #1009

gherrit-pr-id: I3d825b6c6b99277c96d9fd6a6761bdfad2000473

This permits us to remove the `KnownLayout` bounds from
`Ptr::transmute`, and thus to remove `Ptr::transmute_sized` entirely.
This also allows us to remove the hand-rolled impl of `TryFromBytes` for
`ManuallyDrop<T>`, replacing it with an invocation of
`impl_for_transmute_from!`.

gherrit-pr-id: I090715fd9ed6f100fd35513b05054015621ecb9c

Closes #1253

gherrit-pr-id: I569b74086a5f98cda71b4a4131f9ce4f89dcc623

* Implement traits for Cell

Closes #1253

gherrit-pr-id: I569b74086a5f98cda71b4a4131f9ce4f89dcc623

* Remove obsolete safety proofs

In #2408, we simplified the safety precondition of `unsafe_impl!`, but
did not remove safety proofs at call sites made redundant by that
simplification. This commit removes those now-obsolete proofs.

gherrit-pr-id: I70d5aa5ace6bd2e39e679eac7f00a66d4b843d57

All moved proofs test code from the `layout` module, and so belong there
rather than in the crate root.

gherrit-pr-id: I691b42ce8c0c3c6e5990e7684fc66f8f5dd73d85

gherrit-pr-id: I90b578eda7942bd1cf206c08801f5855a0ef30a0

gherrit-pr-id: I046abe912435c8944ace21c207334b002efa9ea7

We remove `simd-nightly` impls for the `arm` target `[u]int8x4_t` types,
which are now just type aliases for `u32` and `i32`.

We also switch from using `wasm32-wasi` to `wasm32-unknown-unknown` in
CI, as the former is less supported (and not available via `rustup` on
this particular nightly).

gherrit-pr-id: I0a19a7789334f41ffedc55b871df9736d4b51484

Closes #2400

gherrit-pr-id: I69f30d9256afe91a6cf7619bec2b37b57ce98880

gherrit-pr-id: I681bacc2fbc00531c8493490b9f982fa4aea307a

gherrit-pr-id: Ide939969b8d0c3bc117aa9f1dba06e6d570f262f

gherrit-pr-id: I855bc98cf1674d02e3750e2fc55810be79795796

This language was made stale by the recent change to `PtrInner`'s
invariants.

gherrit-pr-id: I9e6ee4b276bf7a6ccec7b8cfbb44afd61dd36dfa

This allows us to simplify a number of call sites which don't make use
of every parameter that `impl_block` previously accepted.

gherrit-pr-id: If2c6733f78dbc90d34812985a8c39ebaddb211fe

Read the `#[zerocopy(crate = "...")]` attribute (if present), and use
the provided path in the derive output when referencing items exported
from zerocopy. If the attribute isn't present, then `::zerocopy` will be
used.