wtf, sed?!??

Preserve identifiers in binders where possible, analogous to the support
for destructing existentials in !479.

Fixes #336.

Tej Chajed authored Jul 21, 2020 and Robbert Krebbers committed Jul 21, 2020

When running `iDestruct "H" as (?) "H"`, use the name of the binder in
"H". For example, if "H" has type `∃ y, Φ y`,  we now use `y` as the
name of the variable after freshening. Previously the name was always
the equivalent of running `fresh H`.

The implementation achieves this by forwarding the desired identifier
name through the `IntoExist` typeclass. Identifiers are serialized in
Gallina by using them as the name of a function of type `ident_name :=
unit -> unit`.

Gregory Malecha authored May 29, 2020 and Robbert Krebbers committed May 29, 2020

- it doesn't seem to conflict with anything in Ltac

Here's one case this lemma might be useful. Suppose we want to
programmatically generate namespaces for e.g. locks:

```
Definition lockN (l : loc) := nroot .@ "lock" .@ l.
```

Then to know that two such namespaces are disjoint, we need to know
that the corresponding locations are distinct. For that we use
the lemma here introduced.

```
Lemma ne l1 l2 v1 v2 :
  l1 ↦ v1 -∗ l2 ↦ v2 -∗ ⌜l1 ≠ l2⌝.
Proof. iApply mapsto_mapsto_ne. (* goal ¬ ✓ 2%Qp *) by intros []. Qed.
```