[pre-pitch] Nonisolated nonsending Clock protocol

I'd like to pre-pitch updating the Clock protocol to have a nonisolated(nonsending) sleep, just to get people's first impression. The Clock protocol has one method requirement, and currently it is @concurrent. This means every time clock.sleep is called it will jump to the concurrent executor, whether it needs to or not. Other parts of the standard library have gotten the nonisolated(nonsending) treatment, such as withTaskCancellationHandler and withCheckedContinuation, so why not Clock? And i...