cpp/thread/stop token

The class provides the means to check if a stop request has been made or can be made, for its associated  object. It is essentially a thread-safe "view" of the associated stop-state.

The can also be passed to the constructor of, such that the callback will be invoked if the 's associated std is requested to stop. And can be passed to the interruptible waiting functions of std, to interrupt the condition variable's wait if stop is requested.