Change arena index from `mrb_int` to `int`.

src/load.c

@@ -211,7 +211,7 @@ static mrb_irep*
 read_irep_record(mrb_state *mrb, const uint8_t *bin, size_t *len, uint8_t flags)
 {
   struct RData *irep_obj = mrb_data_object_alloc(mrb, mrb->object_class, NULL, &tempirep_type);
-  mrb_int ai = mrb_gc_arena_save(mrb);
+  int ai = mrb_gc_arena_save(mrb);
   mrb_irep *irep = read_irep_record_1(mrb, bin, len, flags);
   int i;