cpp/thread/stop source

The class provides the means to issue a stop request, such as for std cancellation. A stop request made for one object is visible to all s and stds of the same associated stop-state; any std(s) registered for associated std(s) will be invoked, and any std objects waiting on associated std(s) will be awoken.

Once a stop is requested, it cannot be withdrawn. Additional stop requests have no effect.