This is in its own commit so that we can argue about it independently
of the rest of the PR. I believe this is correct and safe because the
docs for `AtomicPtr` say that (a) it is #[repr(C)] and (b) that it
has the same in-memory representation as a *mut T.

This covers both the std and no-std cases. This is perhaps an unconventional
strategy; see rust-bitcoin#346 for
discussion leading up to it.

Weirdly we have no tests that don't require std or alloc. We should
revisit this. In many cases it is because of the dependency on
rand-std but we should really try to narrow that down to specific
tests.

This commit should probably be removed before merging; it can be part
of the next PR.