Add tuples

[seaneagan]

Could alternatively put these in a separate quiver.tuples library if desired.

Can also increase (or decrease) the maximum tuple length provided (I only need length = 2 right now :) ).

Looking to add this so I can use it to represent Map pairs:

Iterable<Tuple2> pairs(Map map);
Map fromPairs(Iterable<Tuple2> pairs);

And I'd also like to use it to replace IndexedValue:

Iterable<Tuple2<int, dynamic>> enumerate(Iterable iterable);
Stream<Tuple2<int, dynamic>> enumerate(Stream stream);

[justinfagnani]

So we just discussed this PR, and I don't think we want to encourage Pairs/Tuples. Classes are easy enough to create, and we've all seen very nasty uses of them. I wouldn't mind tuples in the language though.