Move away from pointers as usize

Pointers stored as `usize` tend to cause miri to lose pointer provenance
tracking, which means we can't take advantage of its checking! See also
the discussion at
rust-lang/miri#940 (comment).

This does not yet compile since `AtomicPtr` does not have `fetch_*`
methods. They were added and then removed from the standard library back
in the day (rust-lang/rust#10154), but I think
the reason they were removed has now been remedied, so they will
hopefully come back again!

Author: jonhoo