implement AtomicStorage for Optionals of RawRepresentable types

[glessard]

An example I wrote a while back needs atomics both for a RawRepresentable type and its Optional. This particular combination of functionality was missing from the package.

This PR defines an implementation of AtomicStorage for this case:

public struct AtomicOptionalRawRepresentableStorage<Wrapped>: AtomicStorage
  where Wrapped: RawRepresentable,
        Wrapped.RawValue: AtomicOptionalWrappable {
  // ...
}

A RawRepresentable type can semi-automatically conform to AtomicOptionalWrappable by declaring (via typealias) that its AtomicOptionalRepresentation is AtomicOptionalRawRepresentableStorage<Self>.

Checklist

• [x] I've read the Contribution Guidelines
• [x] My contributions are licensed under the Swift license.
• [x] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [x] I've verified that my change does not break any existing tests.
• [x] I've updated the documentation if necessary.

[glessard]

I don't have complete tests yet, but thought I should check in for feedback anyway.

[lorentey]

I suspect testing will uncover an issue with the current definition of RawRepresentable.AtomicOptionalRepresentation, but let's land this on current main before tackling that.