fs: use async fn instead of custom futures (#1381)

Also update all the doc examples.

tokio-fs/src/read_link.rs

@@ -1,46 +1,13 @@
-use std::fs;
-use std::future::Future;
+use crate::asyncify;
+
 use std::io;
 use std::path::{Path, PathBuf};
-use std::pin::Pin;
-use std::task::Context;
-use std::task::Poll;
 
 /// Reads a symbolic link, returning the file that the link points to.
 ///
 /// This is an async version of [`std::fs::read_link`][std]
 ///
 /// [std]: https://doc.rust-lang.org/std/fs/fn.read_link.html
-pub fn read_link<P: AsRef<Path>>(path: P) -> ReadLinkFuture<P> {
-    ReadLinkFuture::new(path)
-}
-
-/// Future returned by `read_link`.
-#[derive(Debug)]
-#[must_use = "futures do nothing unless you `.await` or poll them"]
-pub struct ReadLinkFuture<P>
-where
-    P: AsRef<Path>,
-{
-    path: P,
-}
-
-impl<P> ReadLinkFuture<P>
-where
-    P: AsRef<Path>,
-{
-    fn new(path: P) -> ReadLinkFuture<P> {
-        ReadLinkFuture { path }
-    }
-}
-
-impl<P> Future for ReadLinkFuture<P>
-where
-    P: AsRef<Path>,
-{
-    type Output = io::Result<PathBuf>;
-
-    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
-        crate::blocking_io(|| fs::read_link(&self.path))
-    }
+pub async fn read_link<P: AsRef<Path>>(path: P) -> io::Result<PathBuf> {
+    asyncify(|| std::fs::read_link(&path)).await
 }