QoL: reset the toplevel every time before evaluating code from the ed…

…itor (#411)

You might lose what you manually typed directly in the toplevel pane, but that
shouldn't be a problem, and most importantly it will avoid already defined ids
to clutter the environment.

These often cause student confusion with cases like "type t instead of t", or a
missing `rec` keyword that compiles because using an older definition.

src/app/learnocaml_common.ml

@@ -894,6 +894,7 @@ module Editor_button (E : Editor_info) = struct
   let eval top select_tab =
     editor_button
       ~icon: "run" [%i"Eval code"] @@ fun () ->
+      Learnocaml_toplevel.reset top >>= fun () ->
       Learnocaml_toplevel.execute_phrase top (Ace.get_contents E.ace) >>= fun _ ->
       select_tab "toplevel";
       Lwt.return_unit