reduce size of Response, async::Response, and async::Decoder

src/response.rs

@@ -335,3 +335,30 @@ pub fn new(mut res: async_impl::Response, timeout: Option<Duration>, thread: Kee
         _thread_handle: thread,
     }
 }
+
+#[test]
+fn mem_size_of() {
+    assert_eq!(::std::mem::size_of::<Response>(), 0);
+}
+
+#[test]
+fn mem_size_of_readable_async_res() {
+    assert_eq!(::std::mem::size_of::<async_impl::Response>(), 0);
+}
+
+#[test]
+fn mem_size_of_wait_body() {
+    assert_eq!(::std::mem::size_of::<WaitBody>(), 0);
+}
+
+#[test]
+fn mem_size_of_url() {
+    assert_eq!(::std::mem::size_of::<::Url>(), 0);
+}
+
+#[test]
+fn mem_size_of_readable_chunks_wait_body() {
+    assert_eq!(::std::mem::size_of::<async_impl::ReadableChunks<WaitBody>>(), 0);
+}
+
+