The proof mode now explicitly keeps track of anonymous hypotheses (i.e.
hypotheses that are introduced by the introduction pattern `?`). Consider:

  Lemma foo {M} (P Q R : uPred M) : P -∗ (Q ∗ R) -∗ Q ∗ P.
  Proof. iIntros "? [H ?]". iFrame "H". iFrame. Qed.

After the `iIntros`, the goal will be:

  _ : P
  "H" : Q
  _ : R
  --------------------------------------∗
  Q ∗ P

Anonymous hypotheses are displayed in a special way (`_ : P`). An important
property of the new anonymous hypotheses is that it is no longer possible to
refer to them by name, whereas before, anonymous hypotheses were given some
arbitrary fresh name (typically prefixed by `~`).

Note tactics can still operate on these anonymous hypotheses. For example, both
`iFrame` and `iAssumption`, as well as the symbolic execution tactics, will
use them. The only thing that is not possible is to refer to them yourself,
for example, in an introduction, specialization or selection pattern.

Advantages of the new approach:

- Proofs become more robust as one cannot accidentally refer to anonymous
  hypotheses by their fresh name.
- Fresh name generation becomes considerably easier. Since anonymous hypotheses
  are internally represented by natural numbers (of type `N`), we can just fold
  over the hypotheses and take the max plus one. This thus solve issue #101.

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