[computed] Support async iterators

const Clock = provideState({
  computed: {
    // it simply returns an async iterator instead of a value
    //
    // the computed is undefined before the first
    // value is emitted by the iterator
    //
    // each new emitted value will trigger a new render with the
    // computed set to this value
    //
    // rejections are not handled and will possibly trigger an
    // unhandledRejection event on the window object, the computed will
    // keep its previous value
    //
    // this computed, which does not have dependencies, yields a new
    // date value every second
    async *date() {
      while (true) {
        yield new Date()
        await new Promise(resolve => setTimeout(resolve, 1e3))
      }
    }
  }
})(injectState(({ state }) => <p>{state.date.toISOString()}</p>))

The issue names no files, tests, or entry points. Start by locating the computed-state implementation; done means async iterators yield values into state, trigger renders, remain undefined before the first yield, and retain the previous value after rejection.