BigInt (Stage 3)

Champion: @littledan
Spec Repo: https://github.com/tc39/proposal-bigint
Spec Text: https://tc39.github.io/proposal-bigint
Slides (to Stage 3 at May TC39 Meeting): https://docs.google.com/presentation/d/1lrcjQzIFgdUXczeeAzs4GkTXJsRQU21UhtmXef70Qic/edit#slide=id.p
> moved to Stage 3 at July TC39 Meeting https://github.com/tc39/agendas/blob/master/2017/07.md

The syntax has been implemented in Babylon, see https://github.com/babel/babylon/issues/569#issuecomment-306478858 . What remains is a plugin to implement the transform.

- All arithmetic operations and comparisons should be replaced by calls to functions which provide the appropriate behavior depending whether arguments are BigInts or Numbers.
- The transform is slow, so it will really be more of a toy rather than something that should be used in production. For this reason, I'd argue it should be left out of Babel presets.
- Polyfill needed--ideally, this would be a thin wrapper around another BigNum library from npm. There are just a few methods to support. Some functions will have to be a bit different to get close to the right behavior, for example, `valueOf()` should throw a TypeError, so that implicitly casting to Number throws a TypeError, even if it would really return the BigInt. Additionally, the polyfill needs BigInt64Array and DataView operations.
- A good starting point from @amilajack:
> I've done some work on this in a [babel plugin](https://github.com/amilajack/babel-plugin-fail-explicit) that prevents coercion in JS. Here's the [related code](https://github.com/amilajack/babel-plugin-fail-explicit/blob/master/src/index.js#L127-L155) for that

Start with the linked TC39 BigInt proposal and the Babylon issue, then inspect the related babel-plugin-fail-explicit code in src/index.js around lines 127-155. Done means a BigInt transform handles arithmetic and comparisons, with the described polyfill support including BigInt64Array and DataView operations.

Written by the indexing model from the issue text.