io: implement AsyncBufRead for &[u8] and Cursor (#1397)

* `impl AsyncRead for &[u8]`
* `impl AsyncBufRead for &[u8]`
* `impl<T: AsRef<[u8]> + Unpin> AsyncRead for Cursor<T>`
* `impl<T: AsRef<[u8]> + Unpin> AsyncBufRead for Cursor<T>`

tokio-io/tests/read_to_string.rs

@@ -1,38 +1,13 @@
 #![deny(warnings, rust_2018_idioms)]
 #![feature(async_await)]
 
-use tokio_io::{AsyncRead, AsyncReadExt};
+use tokio_io::AsyncReadExt;
 use tokio_test::assert_ok;
 
-use std::pin::Pin;
-use std::task::{Context, Poll};
-use std::{cmp, io};
-
 #[tokio::test]
 async fn read_to_string() {
-    struct Rd {
-        val: &'static [u8],
-    }
-
-    impl AsyncRead for Rd {
-        fn poll_read(
-            mut self: Pin<&mut Self>,
-            _cx: &mut Context<'_>,
-            buf: &mut [u8],
-        ) -> Poll<io::Result<usize>> {
-            let me = &mut *self;
-            let len = cmp::min(buf.len(), me.val.len());
-
-            buf[..len].copy_from_slice(&me.val[..len]);
-            me.val = &me.val[len..];
-            Poll::Ready(Ok(len))
-        }
-    }
-
     let mut buf = String::new();
-    let mut rd = Rd {
-        val: b"hello world",
-    };
+    let mut rd: &[u8] = b"hello world";
 
     let n = assert_ok!(rd.read_to_string(&mut buf).await);
     assert_eq!(n, 11);