With a pretty proof by Robbert

This restricts CAS to only be able to compare literals with literals, NONEV with NONEV and NONEV with SOMEV for a literal.

* move PROP-envs definitions to environments.v so that we can control them without pulling in coq_tactics
* use reduction-controlled `pm_default` for proofmode accessors

  Also, remove the inconsistency that `wp_expr_eval` succeeds on a goal
  that is not a WP.

We now use the `Maybe` prefix as also used for `Frame`: it indicates
whether progress has been made by stripping of a later or not.

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

This reverts commit 913059d2.

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

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.

This class, in combination with `TCForall`, turns out the useful in
LambdaRust to express that lists of expressions are values.