Start improving control over type class search in proof mode tactics.

We do this in two ways:

- Use `notypeclasses refine` instead of `eapply`, to avoid type class
  search being called arbitrary.
- Use `typeclasses eauto` instead of `apply _`, to avoid type class
  search being called on unrelated evars.

I mainly tried this for `iSpecialize` and friends; this same remains to
be done for all other tactics.

This commit also makes partial progress w.r.t. issue #135.