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

tokio-fs/src/hard_link.rs

@@ -20,6 +20,7 @@ pub fn hard_link<P: AsRef<Path>, Q: AsRef<Path>>(src: P, dst: Q) -> HardLinkFutu
 
 /// Future returned by `hard_link`.
 #[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct HardLinkFuture<P, Q>
 where
     P: AsRef<Path>,