This is very experimental. It should now deal better with stuff like:

  match x with .. end = match y with .. end

In case there is a hypothesis H : R x y, it will try to destruct it.

Now, for example, when having equiv (Some x) (Some y) it will
try to find a Proper whose range is an equiv before hitting the
eq instance. My hack is general enough that it works for Forall2,
dist, and so on, too.

Initial commit by Amin Timany.

This seems to shorten type class search.

Also, slightly reorganize.