Avoid exponential blowup in [done] as caused by a7e91677.

Example:

Goal
  ¬False → ¬False → ¬False → ¬False → ¬False → ¬False → ¬False →
  False.
Proof.
  intros. done. (* takes very long *)

theories/tactics.v

@@ -66,7 +66,7 @@ Ltac done :=
     | discriminate
     | contradiction
     | split
-    | match goal with H : ¬_ |- _ => case H; clear H; done end
+    | match goal with H : ¬_ |- _ => case H; clear H; fast_done end
     ]
   ].
 Tactic Notation "by" tactic(tac) :=