Clean up Subject, Observable, and History interface

[nhunzaker]

1. Remove start callback. This is in compliance with the ES spec.
2. Move Observable.hash to Subject.hash. It returns a Subject.
3. History is now a Subject, eliminating history.updates
4. Update Flow types. I think the new way Observables are structured is a lot nicer, in terms of Flow.

Why

1. Remove start callback. This is in compliance with the ES spec.

Besides being against the spec, I didn't like how Microcosm 12.x has a open and update command. Most of the time you know if an action has been opened. You just pushed it beyond that, I want to encourage next -> complete to be the same data type.

2. Move Observable.hash to Subject.hash. It returns a Subject.

This just sort of made sense

3. History is now a Subject, eliminating history.updates

I want all major pieces to adhere to the observable spec. I can see subscribing to history updates being useful for tooling. But mostly this is just an attempt at hygiene. Ideally, you could eventually do something like.

[codecov-io]

Codecov Report

Merging #481 into master will decrease coverage by 0.02%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #481      +/-   ##
==========================================
- Coverage   97.65%   97.62%   -0.03%     
==========================================
  Files          29       29              
  Lines         681      673       -8     
  Branches      143      143              
==========================================
- Hits          665      657       -8     
  Misses         13       13              
  Partials        3        3

Impacted Files	Coverage Δ
packages/microcosm/src/microcosm.js	84.37% <100%> (+0.5%)	:arrow_up:
packages/microcosm/src/effect-engine.js	100% <100%> (ø)	:arrow_up:
packages/microcosm-dom/src/presenter.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/subject.js	100% <100%> (ø)	:arrow_up:
packages/microcosm-dom/src/action-form.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/domain-engine.js	98.03% <100%> (+0.08%)	:arrow_up:
packages/microcosm/src/observable.js	100% <100%> (ø)	:arrow_up:
packages/microcosm-dom/src/action-button.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/history.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/coroutine.js	100% <100%> (ø)	:arrow_up:

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