Remove AtomicOption

[ghost]

Closes #2.

AtomicOption is unsound and is probably not worth repairing. Let's replace it with AtomicCell from crossbeam-rs/rfcs#28 as soon as possible.

[Vtec234]

Agreed, but currently scoped.rs uses AtomicOption so we might have to wait until new types are added before merging this.

As a side note, if we want to replace AtomicOption with AtomicCell, this would create a dependency of utils on atomic, so another reason to just put that type in utils instead.

[ghost]

Rebased on master, where scoped.rs doesn't depend on AtomicOption anymore.

@Vtec234 I'm beginning to agree... We should address these dependency shenanigans somehow.