Avoid using `mrb_ary_modify` from the internal function.

`mrb_ary_modify` calls `mrb_write_barrier`, so can cause the same
problem of the past `push`. It is provided for use-level API.

src/array.c

@@ -943,7 +943,7 @@ mrb_ary_aset(mrb_state *mrb, mrb_value self)
   mrb_value v1, v2, v3;
   mrb_int i, len;
 
-  mrb_ary_modify(mrb, mrb_ary_ptr(self));
+  ary_modify(mrb, mrb_ary_ptr(self));
   if (mrb_get_argc(mrb) == 2) {
     mrb_value *vs = mrb_get_argv(mrb);
     v1 = vs[0]; v2 = vs[1];