Tactic deprecation machinery

We make it possible to deprecate tactics defined by `Ltac`, `Tactic
Notation` or ML.

For the first two variants, we anticipate the syntax of attributes:
`#[deprecated(since = "XX", note = "YY")]`

In ML, the syntax is:

```
let reflexivity_depr =
  let open CWarnings in
  { since = "8.5"; note = "Use admit instead." }

TACTIC EXTEND reflexivity DEPRECATED reflexivity_depr
  [ "reflexivity" ] -> [ Tactics.intros_reflexivity ]
END
```

A warning is shown at the point where the tactic is used (either
a direct call or when defining another tactic):

Tactic `foo` is deprecated since XX. YY

YY is typically meant to be "Use bar instead.".

1 files changed, 2 insertions, 0 deletions

diff --git a/CHANGES b/CHANGES
index 6dce7b0db..fdd4feb59 100644
--- a/CHANGES
+++ b/CHANGES
@@ -38,6 +38,8 @@ Tactics
   without adding new ones. Preexisting tactic `constr_eq_nounivs` can
   still be used if you really want to ignore universe constraints.
 
+- Tactics and tactic notations now understand the `deprecated` attribute.
+
 Tools
 
 - Coq_makefile lets one override or extend the following variables from