Micromanage inception

[nhunzaker]

This PR starts micromanage, what I consider to be the missing piece of Microcosm: a data management and querying layer. Domains work reasonably well enough as data-bags, however I'm not happy with how low level their data management operations are. Consider:

import { Domain } from 'microcosm'

function getWidget (id) {
  return fetch(`/widgets/${id}`).then(res => res.json)
}

class Widgets extends Domain {
  getInitialState() {
    return []
  }  
  addWidget(widgets, widget) {
    return widgets.concat(widget)
  }
  register() {
    return {
      [getWidget]: this.addWidget
    }
  }  
}

class WidgetShow extends Presenter {
  getModel(repo, { id }) {
    return {
      widget: repo.domains.widgets.map(widgets => widgets.find(w => w.id === id))
    }
  }

  ready(repo, props) {
    repo.push(getWidget, props.id)
  }

  update(repo, props) {
    if (props.id !== this.props.id) {
      repo.push(getWidget, props.id)
    }
  }
}

* Important: This is the microcosm 13 API.

This is too low level:

• You shouldn't have to concatenate a record to a collection on your own when an action resolves.
• You also shouldn't have to worry about fetching data if a property of a component has changed, or deal with that state.
• getModel should be less cumbersome. You should just be able to do something like repo.domains.widgets.find(props.id), or something even shorter.

We can do better. These are not valuable uses of project time. What if this were something like:

import { Entity, Collection } from 'micromanage'

class Widget extends Entity {
  static schema = {
    id: String,
    name: String,
    weight: Number
  }
}

class Widgets extends Collection(Widget) {
  // Intentionally left blank, but you could include your own stuff
}

class WidgetShow extends Presenter {
  getModel(repo, { id }) {
    return {
      widget: repo.domains.widgets.find(1)
    }
  }
}

Querying for an entity is never null. repo.domains.widgets.find above could return:

• A loading state record for the widget
• A widget record
• An error state record for the widget

So here goes! This PR includes an initial outline of what I'd like. It's pretty high level, but I thought I'd send it out for transparency.

[codecov-io]

Codecov Report

Merging #488 into master will increase coverage by 0.14%. The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #488      +/-   ##
========================================
+ Coverage   97.86%    98%   +0.14%     
========================================
  Files          29     29              
  Lines         702    702              
  Branches      137    137              
========================================
+ Hits          687    688       +1     
+ Misses         13     12       -1     
  Partials        2      2

Impacted Files	Coverage Δ
packages/microcosm-dom/src/utilities.js	100% <0%> (+7.14%)	:arrow_up:

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[efatsi]

Exciting!