Remove fancy iterators

[elalish]

This refactor will aid #823 - by removing zip iterators all we'll need is a counting iterator, which will reduce the amount of Thrust we rely on. Frankly, I also feel like this cleans up the code a little anyway - I always hated those tuples.

I've only done a few files so far, but it's going pretty quickly. Should be no big deal - just wanted to show my progress.