…ies of hyps

These are the beginnings of an API about dependencies between expressions. For
now, we only deal with dependencies and reverse dependencies of hypotheses,
which are easy to compute.

Breaking change: `tactic.revert_deps` is renamed to
`tactic.revert_reverse_dependencies_of_hyp` for consistency. Its implementation
is completely new, but should be equivalent to the old one except for the order
in which hypotheses are reverted. (But the old implementation made no particular
guarantees about this order anyway.)

I thought these trivial variants would just clutter the API, but then I
immediately needed a couple of them for another tactic.

Co-authored-by: Rob Lewis <Rob.y.lewis@gmail.com>

…ultiple hyps; improve docs