Adopt swift-atomics as a dependency

[natecook1000]

If we take the approach in #457 of subsetting a sliver of swift-atomics, we should switch to rely on the actual package when we're able to do so. (i.e. when that doesn't pose problems for integration into the compiler, etc.)

[natecook1000]

We went a different route, using existing stdlib methods, to this is no longer necessary.