Improve the debugging infrastructure (cherry-pick from

 parts of 802f291e).

misc/debug.ml

+let margin = ref  ""
+let increase_margin () = margin := !margin ^ "  "
+let decrease_margin () = margin := String.sub !margin 0 (max (String.length !margin - 2) 0)
+
+let  _DEBUG =  fun level ?(enter=false) ?(leave=false) fmt ->
+  let () =
+    if enter then begin
+	Format.eprintf "%s> Entering %s: @\n%!" !margin level;
+	increase_margin ()
+      end
+    else if leave then begin
+	decrease_margin();
+	Format.eprintf "\n%s< Leaving %s:@\n%!" !margin level;
+      end
+  in
+  Format.eprintf "%s" !margin;
+  Format.eprintf fmt

tests/poly/perf-tallying.cd

+let x : (('c -> 'c) & ('b -> 'b) & ('a -> 'a)) = fun ('aa -> 'aa; 'bb -> 'bb ; 'cc -> 'cc ) x -> x ;;
+