* Can be used to specialize the asynchronous control flow
on immediate available asynchronous results mostly returned by:
- make_ready_continuable
- make_exceptional_continuable
* Usable to avoid longer unnecessary synchronous callback nestings.
* cti::query_arg_t was renamed into cti::unpack_arg_t.
* The continuation overload nowreturns result<Args...> rather
than std::tuple<Args...>.
* Add is_ready optimizations to make_exceptional_continuable.
