Experiment: Implement saga-like effects through generators

[gordonbrander]

This PR takes a different approach to store effects, using generators to implement simple feed-forward sagas.

• Store update function simply returns new state (vanilla reducer)
• A new store property fx can be giving an async generator function that generates effects
  • This async generator function is called on every update with the current state and message (just like the update function).
  • The async generator function may yield zero or more effects

Essentially effects are managed separately from state updates. The top-level fx generator function is typically written with a switch statement, just like the update function:

async function* fx(state, msg) {
  switch (msg.type) {
  case 'a':
    yield* aFx(state, msg)
    return
  case 'b':
    yield* bFx(state, msg)
    return
  default:
    return
  }
}

[gordonbrander]

Note that this PR switch to feed-forward async generators. I believe implementing cooperative generators that can communicate bi-directionally and fold with state/msg would require a considerably more complex cooperative multitasking runtime.

[gordonbrander]

Further investigation has revealed that nearly all useful use-cases for generator-style effects requires communication with the messages sent to the store.

I believe doing this correctly will require something like Communicating Sequential Processes so that we can ensure we don't miss relevant messages while a saga is busy doing something else.

Closing for now.