Add Interleave iterator

@sfackler was interested in having this adapter, so I hacked it together.
It's a bit simpler, and avoids a conditional (which presumably would get optimized away anyway but w/e).

It may actually be beneficial to not even do the conversion here but instead just keep the boolean a boolean and convert it where you actually need the integral value (e.g. that way `1 - off_by_one` just becomes `!using_first as uint`).