Get rid of `later_proper'`

See merge request FP/iris-coq!82

add later_ne; make later_proper match the other _proper lemmas

See merge request FP/iris-coq!81

This reverts commit 849abb8d, reversing changes
made to 8de0b894.  The merge was done
accidentally, before review happened.

Add progress bit to WP.

See merge request FP/iris-coq!37

This is derived from `wp_forget_not_stuck` and a trivial preorder on
stuckness bits. (The two lemmas are redundant, but I have examples
where each seems more natural than the other.)

I did *not* bake `wp_stuckness_mono` into `strong_mono` for two
reasons. Mainly, I didn't see a nice way to combine the two proofs
(beyond `cut`). Less important, changing the type of `wp_strong_mono`
will break code.

I saw no need for `stuckness_flip`: strong atomicity always works,
while weak atomicity works only for expressions that are not stuck.
Since this seemed unclear, I split lemma `wp_atomic'` up into
`wp_strong_atomic` (parametric in the WP's `s`) and `wp_weak_atomic`
(not). The proof mode instance is stated in terms of the derived rule
`wp_atomic` (parametric in `s`).

Pull progress bit out of the WP fixpoint,
make (most) wp adequacy notation only parsing, and
generalize forget_progress.

rename sub_values -> sub_redexes_are_values

See merge request FP/iris-coq!79

Make atomic a type class.

See merge request FP/iris-coq!78

Thanks to Amin Timany for an initial version of the proof.

Make proofnode notation more robust in the context of import order

See merge request FP/iris-coq!75

This solves issue #100: the proof mode notation is sometimes not printed. As
Ralf discovered, the problem is that there are two overlapping notations:

```coq
Notation "P ⊢ Q" := (uPred_entails P Q).
```

And the "proof mode" notation:

```
Notation "Γ '--------------------------------------' □ Δ '--------------------------------------' ∗ Q" :=
  (of_envs (Envs Γ Δ) ⊢ Q%I).
```

These two notations overlap, so, when having a "proof mode" goal of the shape
`of_envs (Envs Γ Δ) ⊢ Q%I`, how do we know which notation is Coq going to pick
for pretty printing this goal? As we have seen, this choice depends on the
import order (since both notations appear in different files), and as such, Coq
sometimes (unintendedly) uses the first notation instead of the latter.

The idea of this commit is to wrap `of_envs (Envs Γ Δ) ⊢ Q%I` into a definition
so that there is no ambiguity for the pretty printer anymore.