Add Interleave iterator

@sfackler was interested in having this adapter, so I hacked it together.
Honestly it struck me as weird to even *bother* with these asserts. Not trusting something that claims to implement an API to implement it strikes me as way over-defensive. But I figured if Zip does it, I probably should.

I could write up a PR to add it if you're honestly interested though, since I'm hacking away in here already.