add #[must_use] to more futures and streams (#1309)

tokio-fs/src/write.rs

@@ -41,6 +41,7 @@ where
 /// A future used to open a file for writing and write the entire contents
 /// of some data to it.
 #[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct WriteFile<P: AsRef<Path> + Send + Unpin + 'static, C: AsRef<[u8]> + Unpin> {
     state: State<P, C>,
 }