Possible specialisations for inbox

[Thomasdezeeuw]

Just some thought on possible specialisations.

Specialise for small types, less than usize, that can be atomically written.

Specialise for non-zero types, e.g. Box, Rc, Arc (via NonNull) and NonZeroUsize, this would allow the zero value to be empty.

Both optimisations likely require specialization: https://github.com/rust-lang/rust/issues/31844.