Add AtomicInt and AtomicUInt types

[jplatte]

Would be potentially useful for state-res, which since today uses an AtomicU64 in its tests and benchmarks (previously was using a static mut 😬).

cc @DevinR528

[DevinR528]

Sweet! I didn't know those were const yet, good to know. I'll keep my eye out to see if there are any more uses but I bet we could find some 😄

[FSMaxB]

I've independently looked into this once already. The main issue here is that you can't have fetch_add and fetch_sub with regular memory orderings, it needs to be implemented with a compare_exchange internally in order to not violate the invariants of the types, meaning you need separate load and store orderings.

Other than that annoyance of a slightly more complicated API, it's totally feasible to implement!