Add map2...map5 for Future and Future of Result

[mlms13]

These functions allow you to wait for multiple futures to resolve and "map" the separate values from all of those futures into a new value.

Here is the corresponding functionality for the Task type in Elm as well as a similar idea in this promise library for Haxe.

I chose to use the mapN name (as Elm does) because it seems more easily-understandable than a name like liftA2 used in Haskell. I also chose to implement map5 in terms of map4 mostly because it's much more terse than lots of nested flatMaps, but the code might be a little less readable as a result.

[coveralls]

Pull Request Test Coverage Report for Build 29

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 98.387%

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Totals
Change from base Build 26:	0.4%
Covered Lines:	109
Relevant Lines:	110

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 29

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 98.387%

---

Totals
Change from base Build 26:	0.4%
Covered Lines:	109
Relevant Lines:	110

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 29

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 98.387%

---

Totals
Change from base Build 26:	0.4%
Covered Lines:	109
Relevant Lines:	110

---

💛 - Coveralls

[seprich]

Have you compared your work with the Js.Promise equivalents all2,.. all6 in https://bucklescript.github.io/bucklescript/api/Js.Promise.html ?
The nice thing about the standard bucklescript implementation is the fact that it can be chained with the arrow operator because tuples are used. Also it kind of sets precedent for the preferable naming in my mind but that can be debated. But the main difference is the signature.
Otherwise, especially the mapOk2, etc (or allOk2, ...) functions are IMO very important for effective use of this lib.

[mlms13]

@seprich it would be pretty easy (and probably very worthwhile) to add all2...all6 written in terms of map2...map6, but i think the mapN functions are valuable enough on their own (and a pretty common fixture in functional libraries in general... there's been talk of adding similar mapN functions to Belt.Option and Belt.Result).

Here's a case where it's very useful to have function-of-arity-n instead of tuple-n:

type userWithOrders = {
  user: User.t,
  orders: list(Order.t)
};

let make = (user, orders) => { user, orders };

let futureOfUserWithOrders =
  Future.mapOk2(getUserById("abc"), getOrdersForUserId("abc"), make);

If we want, I'm happy to add several allN functions, which would basically be defined as:

let all3 = (fa, fb, fc) => map3(fa, fb, fc, (a, b, c) => (a, b, c));

[gilbert]

This looks great. It's a useful utility that one shouldn't have to write on their own. Thanks!