Move some [Require]s to the top of a file

It's bad practice to have them in the middle of the file.  Also, it
makes my coq-bug-finder run afoul of
https://coq.inria.fr/bugs/show_bug.cgi?id=3556 when tracking down a bug
in [apply] in the new version of trunk with #440.
Strip out a duplicate [Require Import]
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