V3 unioniterator

supercedes #65

The bolded performance regression needs to be ironed out first.

Before For loop with 5x10^9 elems: 5.207s UnionIterator 2x10^7 iterators: 30.297s UnionIterator 1000x500 iterators: 1.897s UnionIterator 1000x500 iterators - max parallelism of 1: 1.837s

After For loop with 5x10^9 elems: 6.349s UnionIterator 2x10^7 iterators: 3.513s UnionIterator 1000x500 iterators: 7.219s UnionIterator 1000x500 iterators - max parallelism of 1: 168.007ms