`Ptr` is like `NonNull`, but has many restrictions which make it so that
using a `Ptr` in unsafe code requires much simpler soundness proofs. In
particular, `try_cast_into` attempts to cast a `Ptr<[u8]>` into a
`Ptr<U>` where `U: ?Sized + KnownLayout`, and will be built upon in
future commits as a building block of `TryFromBytes`.

Because `try_cast_into` performs a runtime check to validate alignment,
this requires disabling Miri's "symbolic alignment check" feature. While
it would be possible to run both with and without that feature in order
to still test other code using symbolic alignment checking, I don't
think that the benefit is worth a) the complexity of passing the
information necessary for certain tests to not run under symbolic
alignment checking and, b) the extra CI time to run Miri tests twice.

Makes progress on #29