Combines multiple iterators.

When this iterator is advanced, it advances each sub-iterator, collects all of their results into a newly-allocated, mutable array, and produces that array.

This iterator finishes as soon as any one of its sub-iterators is finished.

(let it (zip "abcde" "12" "ABCD"))
(prn ..it) ; prints (\a 1 \A) (\b 2 \B)