decide state based on the returned value of starting plan (WIP)
decide state based on the returned value of starting plan (WIP)

- when plan decides to reject, just change state to REJECTED. no exception
- when plan decides to resolve,
    - primitive value should pass and makes promise fullfiled
    - Promise or promise-like (has callable `then` property) object should be
      - executed with plan (host pauses resolving)
      - (host resume resolving) and resolve the plan output
    - it does not actually pause the resolving, just inject the destination
      of plan back to host's resolver/rejector

chintown/simple-promise,chintown/simple-promise