feat(library/init/meta/tactic): add timetac combinator

library/init/meta/tactic.lean

@@ -196,6 +196,9 @@ do fmt ← pp a,
 meta def trace_call_stack : tactic unit :=
 take state, _root_.trace_call_stack (success () state)
 
+meta def timetac {α : Type u} (desc : string) (t : tactic α) : tactic α :=
+λ s, timeit desc (t s)
+
 meta def trace_state : tactic unit :=
 do s ← read,
    trace $ to_fmt s