add AtomicOption

[programmerjake]

Add AtomicOption, an atomic version of Option<ReferenceLike<T>> where ReferenceLike<T> is Box<T>, Arc<T>, &T, and &mut T for now -- basically anything that has a lay-out like NonNull<T>.

I didn't implement AtomicOption<NonNull<T>> for now as NonNull<T> is not Send so AtomicOption is mostly useless for that case.

[ghost]

Hi @programmerjake, and thanks for the pull request! :)

We've recently added AtomicCell to crossbeam-utils, which is very similar to AtomicOption and more general. For example, the equivalent of AtomicOption<Box<T>> would be AtomicCell<Option<Box<T>>>.

See crossbeam-rs/crossbeam-utils#13 and crossbeam-rs/crossbeam-utils#39 for more details.

Instead of adding AtomicOption, we should just publish a new version of crossbeam-utils and then reexport AtomicCell in crossbeam.

What do you think - does AtomicCell have everything you need from it?

[programmerjake]

AtomicCell seems satisfactory. I do think it would be nice to add methods to AtomicCell that allow use of more relaxed atomics:

• Ordering::AcqRel should be fine from safe code as that's what Mutex provides.
• weaker orderings would be nice to have but they would be unsafe unless you can prove that the data doesn't contain any references or pointers, maybe using an auto trait to require that.