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

Also update all the doc examples.

tokio-fs/src/stdin.rs

@@ -1,6 +1,8 @@
+use crate::blocking_io;
+
 use tokio_io::AsyncRead;
 
-use std::io::{self, Read, Stdin as StdStdin};
+use std::io::{self, Read};
 use std::pin::Pin;
 use std::task::Context;
 use std::task::Poll;
@@ -22,7 +24,7 @@ use std::task::Poll;
 /// [`AsyncRead`]: trait.AsyncRead.html
 #[derive(Debug)]
 pub struct Stdin {
-    std: StdStdin,
+    std: std::io::Stdin,
 }
 
 /// Constructs a new handle to the standard input of the current process.
@@ -34,21 +36,12 @@ pub fn stdin() -> Stdin {
     Stdin { std }
 }
 
-impl Read for Stdin {
-    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-        crate::would_block(|| self.std.read(buf))
-    }
-}
-
 impl AsyncRead for Stdin {
     fn poll_read(
-        self: Pin<&mut Self>,
+        mut self: Pin<&mut Self>,
         _cx: &mut Context<'_>,
         buf: &mut [u8],
     ) -> Poll<io::Result<usize>> {
-        match Pin::get_mut(self).read(buf) {
-            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => Poll::Pending,
-            other => Poll::Ready(other),
-        }
+        blocking_io(|| (&mut self.std).read(buf))
     }
 }