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ARROW-5762: [JS] Align Map type impl with the spec

This PR closes [ARROW-5762](https://issues.apache.org/jira/browse/ARROW-5762) by aligning the Map implementation with the spec, enabling its inclusion in the integration tests.

Feature-wise, the biggest change is to the `Struct` and `Map` rows. `Row` is now an abstract base class extended by `StructRow` and `MapRow`.

Row implements [JS's native Map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map) interface (and will pass `row instanceof Map` checks). To align with JS Maps, the `Symbol.iterator` return type changes from `IterableIterator<TValue>` to `IterableIterator<[TKey, TValue]>`.

`Struct` fields are uniform and known in advance, whereas `Map` fields vary by row. The `StructRow` will still take advantage of the `Object.create()` technique to create fast instances using a single `StructRow` instance as its prototype.

However `MapRow` must either be dynamic or have its fields defined on construction, so I've changed `MapRow` to return an ES6 Proxy if available ([supported everywhere except IE11](https://caniuse.com/#search=Proxy)) and fall back to `Object.defineProperties()` if not.

Closes #5371 from trxcllnt/ARROW-5762/map-type and squashes the following commits:

3a0eca91d <ptaylor> ensure generated strings are unique
da2a1ed77 <ptaylor> Merge branch 'master' into ARROW-5762/map-type
58f244aa3 <ptaylor> reimplement Map type

Authored-by: ptaylor <paul.e.taylor@me.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>