This reverts commit 78ba9509.

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 patch was created using

  find -name *.v | xargs -L 1 awk -i inplace '{from = 0} /^From/{ from = 1; ever_from = 1} { if (from == 0 && seen == 0 && ever_from == 1) { print "Set Default Proof Using \"Type*\"."; seen = 1 } }1 '

and some minor manual editing

This fixes the following issue by JH Jourdan:

  The fact of including uPred_[...] in the module uPred (in base_logic.v),
  implies that typeclasses instances are declared twice. Once in module
  uPred and once in module uPred_[...]. This has the unfortunate
  consequence that it has to backtrack to both instances each time the
  first one fails, making failure of type class search for e.g.
  PersistentP potentially exponential.

  Goal ((□ ∀ (x1 x2 x3 x4 x5: nat), True -∗ True) -∗ True : iProp Σ).
    Time iIntros "#H".
    Undo.
    Remove Hints uPred_derived.forall_persistent : typeclass_instances.
    Time iIntros "#H".

Thanks to Jason Gross @ Coq club for suggesting this fix.