* Introduce cancellation_result to represent a cancelled async task
* Add cancellation unit tests
* This doesn't allow cancellation of continuables, it is meant
for treating the special state action canceled on the receiver side.
Cancellation of a chain is still up to the user.
* This makes it possible to use promise_base for optional
promises directly rather than wrapping it as optional<promise_base<...>>.
* Invalidate the promise_base after its first usage.
* Expose an `operator bool()` to make the validility accessible.
* Remove the no longer needed private promise_no_init_arg_t tag.
