Upgrade flow; solidify Record and RecordInstance a bit

[rsolomon]

This pull request aims to help out a bit with issues #1240 and #1172 . It does 3 things:

1. Exposes RecordInstance for import
2. Changes the return type of some chainable functions to return this & T instead of just this
3. Uses the new $ElementType<T, K> for some better type safety

Note that I cannot seem to get $ElementType<T, K> to check set properly, but it works fine for get. I really don't know why it wouldn't work the same, so any assistance would be appreciated.

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[skeet70]

Can someone please merge this before releasing 4.0? Can confirm that even just exporting RecordInstance alone solves all our project's issues with code like:

type StateT = Record<{
  reportData: Map<string, List<Object>>,
  loadingSummary: boolean,
  loadingReport: boolean
}>;
const State = Record({
  reportData: Map(),
  loadingSummary: false,
  loadingReport: false
});

when changed to:

import type { RecordInstance } from 'immutable';

type StateT = RecordInstance<{
  reportData: Map<string, List<Object>>,
  loadingSummary: boolean,
  loadingReport: boolean
}>;
const State = Record({
  reportData: Map(),
  loadingSummary: false,
  loadingReport: false
});

[nanot1m]

About set look here facebook/flow#4804. There is a workaround

[leebyron]

Solid improvements, thanks for the help!