Previously, this code produced a &mut[u8] and a Box<[u8]> to the shared
allocation upon cloning it. If the underlying allocation were actually
shared, such as through a &[u8] from the Deref impl, creating either of
these types incorrectly asserted uniqueness of the allocation.
This fixes the example in #522, but Miri still does not pass on this
test suite with -Zmiri-tag-raw-pointers because Miri does not currently
understand int to pointer casts.
