Support unsigned primitives

[ndkoval]

Support AtomicUInt, AtomicULong, and similar atomic unsigned primitives.

[lppedd]

Any chance unsigned variants get some love?

They can be useful, but if there is not enough general support it's difficult to see people adopting them. More support in kotlinx libs would help.

[lppedd]

Although... I've just experimented a little bit with unsigned numbers in a multiplatform environment.

I'm not sure we want the introduced overhead when dealing with atomics. U* are value classes, but the underlying value is still guarded by method calls / constructor calls.