Simultaneous min & max

There were two more suggestions in the pull request. I added a cleaned-up version of the two-at-a-time loop. Then I added, behind a #conditional, a four-at-a-time loop.

I just removed the "??" elegantly when I had to stick them back in. An earlier post here suggested there's a diminishing-returns effect for multi-element reading, so four-at-a-time may not be worth it. Unless it helps by a big amount, I'm inclined to just remove it.