Rewrite RxJS 6 types

[lgeiger]

This is a complete rewrite of the RxJS 6.3.3 flow typings. The typings were largely generated from the original typescript definitions using dts-bundle and flowgen.
In addition, the typescript definition test are used to validate the definitions.

Also checkout https://github.com/lgeiger/rxjs-flow for the code used during the development of the typings. These typings are currently used in nteract/nteract#3399 but it would be great to get more feedback from other users.

Closes #2617
Closes #2384
Closes #2356
Closes #2285
Closes #2272

Closes #2637
Closes #2616
Closes #2604

[lgeiger]

I'm really sorry for the huge diff. I still hope someone dares to check it out 😇

[lgeiger]

Any ideas why CI fails?

Unexpected file: rxjs_v6.x.x.js. This directory can only contain test files or a libdef file named `rxjs_v6.x.x.js`.

[gantoine]

@lgeiger One of your files definitions/npm/rxjs_v6.x.x/flow_v0.72.x-/test_zip.ts ends in .ts, it might be tripping it up.

[rgbkrk]

How's this now @gantoine?

[lgeiger]

Once this is merged. I'm happy to also upstream our redux-observable@1.0.0 typings.

[gantoine]

@villesau @AndrewSouthpaw Would either of you have experience with rxjs defs? These changes are above my understanding 😅

[rgbkrk]

If it helps -- @jayphelps, one of the leads on RxJS, also helped review these changes along with Lukas in a pairing session to get this all set with @nteract. Same goes for the redux-observable typings (which Lukas can introduce in a new PR).

[gantoine]

Indeed that does help! In that case I'm happy to merge this in. Specs are passing and the formats look good. Great work @lgeiger and others!