tokio-fs: rewrite std echo example using async/await (#1442)

This PR fixes the echo example in tokio-fs.

Refs: #1255

tokio-fs/examples/std-echo.rs

@@ -0,0 +1,34 @@
+//! Echo everything received on STDIN to STDOUT and STDERR.
+#![feature(async_await)]
+
+use futures_util::{FutureExt, SinkExt, StreamExt, TryFutureExt};
+
+use tokio::codec::{FramedRead, FramedWrite, LinesCodec, LinesCodecError};
+use tokio::future::ready;
+use tokio_fs::{stderr, stdin, stdout};
+use tokio_threadpool::Builder;
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let pool = Builder::new().pool_size(1).build();
+
+    pool.spawn(
+        async {
+            let mut input = FramedRead::new(stdin(), LinesCodec::new());
+            let mut output = FramedWrite::new(stdout(), LinesCodec::new());
+            let mut error = FramedWrite::new(stderr(), LinesCodec::new());
+
+            while let Some(line) = input.next().await {
+                let line = line?;
+                output.send(format!("OUT: {}", line)).await?;
+                error.send(format!("ERR: {}", line)).await?;
+            }
+            Ok(())
+        }
+            .map_err(|e: LinesCodecError| panic!(e))
+            .then(|_| ready(())),
+    );
+
+    pool.shutdown_on_idle().await;
+    Ok(())
+}