Implement Object#freeze

src/array.c

@@ -108,6 +108,10 @@ ary_fill_with_nil(mrb_value *ptr, mrb_int size)
 static void
 ary_modify(mrb_state *mrb, struct RArray *a)
 {
+  if (RBASIC_FROZEN_P(a)) {
+    mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen array");
+  }
+
   if (ARY_SHARED_P(a)) {
     mrb_shared_array *shared = a->aux.shared;