[micromanage] Add schemas

[nhunzaker]

This commit adds an initial draft of schemas. If an entity is created without a schema, it fails. Additionally, if a property does not match the schema, it fails. Essentially:

class Planet extends Entity {
  static schema = {
    name: String
  }
}

let earth = new Planet({ name: 'earth' })
let invalid = new Planet({ name: 2 })
 // Raises exception:
// 'Entity "Planet" expected a String for key "name". Instead found Number.'

Is this too strict?

I like the strictness, but I also know that it would be a bummer for an API to change a string or number field, or decide to return null. I still need to figure out the best approach for that.

One idea is to allow it, with an option to send warnings about schema mismatches in a backchannel. You'd get an alert about the issue, but a user wouldn't see an error.

Will this hold up?

This is probably okay for numbers, strings, and booleans. What about dates?

Instead, we could use JSON schemas:

class Planet extends Entity {
  static schema = {
    required: [ "name"],
    name: {
      type: "string",
      description: "name of planet; required."
    }
  }
}

Long term, I can see nice interop with React components that generate forms based on schemas:

• https://github.com/mozilla-services/react-jsonschema-form
• https://github.com/jaredpalmer/formik

Or documentation generation:

• https://github.com/cloudflarearchive/json-schema-docs-generator

[codecov-io]

Codecov Report

Merging #489 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #489   +/-   ##
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  Coverage      98%    98%           
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  Files          29     29           
  Lines         702    702           
  Branches      137    137           
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  Hits          688    688           
  Misses         12     12           
  Partials        2      2

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[efatsi]

Kewl

[nhunzaker]

Thanks, @efatsi!