Don't match on pairs where we don't need to

It's slightly better style, I think, to do it this way.  Also, when we
move to primitive projections, this code will break (see
https://coq.inria.fr/bugs/show_bug.cgi?id=3558).
Merge pull request #446 from JasonGross/no-match-pair

Don't match on pairs where we don't need to