We refer to the unsized tail of a type, but we hadn't defined it.
Let's do that and link to the definition.

We guarantee various things about the layout of pointers and
references, but we'd made no guarantees of equivalence between two
pointers to distinct unsized types that differ in the unsized tail.
Let's make some useful guarantees about this.

Our rules for `as` casts document which pointer-to-pointer casts are
valid.  For unsized types, this validity is based on the compatibility
of the pointer metadata.  Using this and our layout equivalences, we
can define when a pointer transmute will produce the same pointer
value as would a cast.  Let's define that.

We hadn't documented when it's sound to transmute a `Box<T>` to a
`Box<U>`.  Now that we've documented when `*mut T` and `*mut U` can be
assumed to have the same layout, let's use that to document when this
transmute is sound.

The validity rule for the metadata of a wide reference, `Box<T>`,
or raw pointer mentions `dyn Trait` and slice but had omitted
`str`.  Let's fix that.

We document that, for references and `Box<T>`, pointed-to values
with slice or `str` metadata must be no larger than `isize::MAX`.  We
hadn't required this for pointed-to values with `dyn` metadata.  It's
tempting to think this isn't necessary since we separately require
that the metadata point to a vtable generated by the compiler, which
ensures the encoded size of the erased type is OK.

But the bound is on the total size of the pointed-to value, including
any sized prefix of a type with an unsized tail.  Since the prefix
combined with the size in the vtable can push us past the limit, we
need the separate restriction.

Let's apply the rule to both cases and add an admonition to remind
ourselves of why this is needed.