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.".

2 files changed, 9 insertions, 0 deletions

diff --git a/test-suite/output/Deprecation.out b/test-suite/output/Deprecation.out
new file mode 100644
index 000000000..7e290847c
--- /dev/null
+++ b/test-suite/output/Deprecation.out
@@ -0,0 +1,3 @@
+File "stdin", line 5, characters 0-3:
+Warning: Tactic foo is deprecated since X.Y. Use idtac instead.
+[deprecated-tactic,deprecated]
diff --git a/test-suite/output/Deprecation.v b/test-suite/output/Deprecation.v
new file mode 100644
index 000000000..04d5eb3d4
--- /dev/null
+++ b/test-suite/output/Deprecation.v
@@ -0,0 +1,6 @@
+#[deprecated(since = "X.Y", note = "Use idtac instead.")]
+ Ltac foo := idtac.
+
+Goal True.
+foo.
+Abort.