task mechanism

[nolleh]

need coroutine wrapper

[nolleh]

there is more features for achieve same role. do some research, then decide which feature is good to be chosen.

cf. https://stackoverflow.com/questions/63360248/where-is-stdfuturethen-and-the-concurrency-ts https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0443r14.html