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

Also update all the doc examples.

tokio-fs/src/stdout.rs

@@ -1,6 +1,8 @@
+use crate::blocking_io;
+
 use tokio_io::AsyncWrite;
 
-use std::io::{self, Stdout as StdStdout, Write};
+use std::io::{self, Write};
 use std::pin::Pin;
 use std::task::Context;
 use std::task::Poll;
@@ -16,7 +18,7 @@ use std::task::Poll;
 /// [`AsyncWrite`]: trait.AsyncWrite.html
 #[derive(Debug)]
 pub struct Stdout {
-    std: StdStdout,
+    std: std::io::Stdout,
 }
 
 /// Constructs a new handle to the standard output of the current process.
@@ -28,33 +30,17 @@ pub fn stdout() -> Stdout {
     Stdout { std }
 }
 
-impl Write for Stdout {
-    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        crate::would_block(|| self.std.write(buf))
-    }
-
-    fn flush(&mut self) -> io::Result<()> {
-        crate::would_block(|| self.std.flush())
-    }
-}
-
 impl AsyncWrite for Stdout {
     fn poll_write(
-        self: Pin<&mut Self>,
+        mut self: Pin<&mut Self>,
         _cx: &mut Context<'_>,
         buf: &[u8],
     ) -> Poll<io::Result<usize>> {
-        match Pin::get_mut(self).write(buf) {
-            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => Poll::Pending,
-            other => Poll::Ready(other),
-        }
+        blocking_io(|| (&mut self.std).write(buf))
     }
 
-    fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
-        match Pin::get_mut(self).flush() {
-            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => Poll::Pending,
-            other => Poll::Ready(other),
-        }
+    fn poll_flush(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        blocking_io(|| (&mut self.std).flush())
     }
 
     fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {