disclose mrb_top_self() and use it in mruby.c

include/mruby.h

@@ -349,6 +349,7 @@ mrb_state* mrb_open_allocf(mrb_allocf);
 void mrb_close(mrb_state*);
 int mrb_checkstack(mrb_state*,int);
 
+mrb_value mrb_top_self(mrb_state *);
 mrb_value mrb_run(mrb_state*, struct RProc*, mrb_value);
 
 mrb_value mrb_p(mrb_state*, mrb_value);

tools/mruby/mruby.c

@@ -130,9 +130,9 @@ main(int argc, char **argv)
       codedump_all(mrb, n);
 
     if (!args.check_syntax) {
-      mrb_run(mrb, mrb_proc_new(mrb, mrb->irep[n]), mrb_nil_value());
+      mrb_run(mrb, mrb_proc_new(mrb, mrb->irep[n]), mrb_top_self(mrb));
       if (mrb->exc) {
-	mrb_funcall(mrb, mrb_nil_value(), "p", 1, mrb_obj_value(mrb->exc));
+	mrb_p(mrb, mrb_obj_value(mrb->exc));
       }
     }
   }