[microcosm] Always, synchronously, emit the first observer.next()

[nhunzaker]

This PR updates Observable such that the first "next" callback invokes immediately. This is important for Observable::map, particularly when binding data in React. Otherwise, React renders before the scheduler clears the queue for an observable. So you'll end up with something like:

class UserShow extends Presenter {
  getModel(repo, { id }) {
    let { users } = repo.domains

    return {
      user: users.map(users => find(users, { id })
    }
  }
  render() {
    console.log(this.model.user) 
    // Always undefined initially because the observable callback is still
    // in the scheduler
  }
}

Essentially, this makes it so that you can always get a value immediately, even if the initial payload is empty.

[codecov-io]

Codecov Report

Merging #516 into master will decrease coverage by 0.25%. The diff coverage is 90%.

@@            Coverage Diff             @@
##           master     #516      +/-   ##
==========================================
- Coverage   98.78%   98.52%   -0.26%     
==========================================
  Files          30       30              
  Lines         738      748      +10     
  Branches      147      150       +3     
==========================================
+ Hits          729      737       +8     
- Misses          9       11       +2

Impacted Files	Coverage Δ
packages/microcosm/src/domain.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/observable.js	94.54% <88.88%> (-1.54%)	:arrow_down:

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[efatsi]

👍 thanks for all the inline comments on the more obscure things

[nhunzaker]

Thanks, guys! @zporter I've added a note about that case, as you requested.