Make invariants closed under weakening.

See merge request FP/iris-coq!85

This partially solves #112.

Consistently name `wp_value_inv`

See merge request FP/iris-coq!108

`iEval  ... in ...` for performing a tactic in an invidual proofmode hypothesis

Closes #116

See merge request FP/iris-coq!105

This is to make sure that e.g. `//` in `iEval (rewrite .. // ..)` does
not immediately close the goal by reflexivity.

This should fix iris-examples.

We already used the following naming convention: `wp_value'` is stated in
terms of `of_val` and `wp_value` is stated in terms of `IntoVal`. This
commit applies this convention to `wp_value_inv` as well.

Special proof mode class for adding a modality to a goal

See merge request FP/iris-coq!107

This used to be done by using `ElimModal` in backwards direction. Having
a separate type class for this gets rid of some hacks:

- Both `Hint Mode`s in forward and backwards direction for `ElimModal`.
- Weird type class precedence hacks to make sure the right instance is picked.
  These were needed because using `ElimModal` in backwards direction caused
  ambiguity.