vm.c: restore visibility when reopen class/module definition

Ideally, the default visibility should be stored in the scope. But for
the time being mruby stores the visibility in the class/module. As a
result, nesting class/module reopening or class_eval/module_eval could
cause incompatibility. We will try to fix them in the future.

src/vm.c

@@ -1011,6 +1011,7 @@ mrb_mod_module_eval(mrb_state *mrb, mrb_value mod)
   if (mrb_get_args(mrb, "|S&", &a, &b) == 1) {
     mrb_raise(mrb, E_NOTIMP_ERROR, "module_eval/class_eval with string not implemented");
   }
+  MRB_SET_VISIBILITY(mrb_class_ptr(mod), MRB_METHOD_PUBLIC_FL);
   return eval_under(mrb, mod, b, mrb_class_ptr(mod));
 }
 
@@ -2940,6 +2941,9 @@ RETRY_TRY_BLOCK:
       struct RClass *c = mrb_class_ptr(recv);
       const mrb_irep *nirep = irep->reps[b];
 
+      /* restore visibility */
+      MRB_SET_VISIBILITY(c, MRB_METHOD_PUBLIC_FL);
+
       /* prepare closure */
       struct RProc *p = mrb_proc_new(mrb, nirep);
       p->c = NULL;