The concurrency runtime now deploys back to macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, which corresponds to the 5.1 release of the stdlib.
Adjust macro usages accordingly.
The prior implementation of `Task.sleep()` effectively had two
different atomic words to capture the state, which could lead to cases
where cancelling before a sleep operation started would fail to
throw `CancellationError`. Reimplement the logic for the cancellable
sleep with a more traditional lock-free approach by
packing all of the state information into a single word, where we
always load, figure out what to do, then compare-and-swap.
