buf: Inital pass at updating BufStream (#1355)

tokio-buf/tests/buf_stream.rs

@@ -1,5 +1,8 @@
 use tokio_buf::BufStream;
 
-// Ensures that `BufStream` can be a trait object
-#[allow(dead_code)]
-fn obj(_: &mut dyn BufStream<Item = u32, Error = ()>) {}
+#[test]
+fn object_safe() {
+    // Ensures that `BufStream` can be a trait object
+    #[allow(dead_code)]
+    fn obj(_: &mut dyn BufStream<Item = u32, Error = ()>) {}
+}