Change union, intersection and difference to take an iterator of iterators

[Jussi Kalliokoski]

Currently the operations take one iterator as this and another as second. This breaks the "data as this" idea, and is also poorly extensible to support more than two iterators.

I propose changing them to take an iterarator of iterators instead:

[[1,2,3,4,6,7], [2,4,6]]::difference(sub) // yields 1, 3, 7

Then we'd throw when more than two iterators are provided for now. This is also how I'd like to do other set operations, such as cartesian product and concatenation.

WDYT?

[Gaetano Checinski]

@jussi-kalliokoski, I think thats a good idea, i've started implementing zip the same way you proposed here...

[Jussi Kalliokoski]

@nikhedonia 👍