fn interleave_pending(self) -> InterleavePending<Self>
    where
        Self: Sized,
    {
        InterleavePending::new(self)
    }
Introduces an extra [`Poll::Pending`](futures_core::task::Poll::Pending)
in between each read of the reader.

# Examples

```
use core::pin::pin;

use futures::task::Poll;
use futures::io::{AsyncRead, Cursor};
use futures_test::task::noop_context;
use futures_test::io::AsyncReadTestExt;

let reader = Cursor::new(&[1, 2, 3]).interleave_pending();
let mut reader = pin!(reader);

let mut cx = noop_context();

let mut buf = [0, 0];

assert_eq!(reader.as_mut().poll_read(&mut cx, &mut buf[..])?, Poll::Pending);
assert_eq!(reader.as_mut().poll_read(&mut cx, &mut buf[..])?, Poll::Ready(2));
assert_eq!(buf, [1, 2]);
assert_eq!(reader.as_mut().poll_read(&mut cx, &mut buf[..])?, Poll::Pending);
assert_eq!(reader.as_mut().poll_read(&mut cx, &mut buf[..])?, Poll::Ready(1));
assert_eq!(buf, [3, 2]);
assert_eq!(reader.as_mut().poll_read(&mut cx, &mut buf[..])?, Poll::Pending);
assert_eq!(reader.as_mut().poll_read(&mut cx, &mut buf[..])?, Poll::Ready(0));

# Ok::<(), std::io::Error>(())
```

## `AsyncBufRead`

The returned reader will also implement `AsyncBufRead` if the underlying reader does.

```
use core::pin::pin;

use futures::task::Poll;
use futures::io::{AsyncBufRead, Cursor};
use futures_test::task::noop_context;
use futures_test::io::AsyncReadTestExt;

let reader = Cursor::new(&[1, 2, 3]).interleave_pending();
let mut reader = pin!(reader);

let mut cx = noop_context();

assert_eq!(reader.as_mut().poll_fill_buf(&mut cx)?, Poll::Pending);
assert_eq!(reader.as_mut().poll_fill_buf(&mut cx)?, Poll::Ready(&[1, 2, 3][..]));
reader.as_mut().consume(2);
assert_eq!(reader.as_mut().poll_fill_buf(&mut cx)?, Poll::Pending);
assert_eq!(reader.as_mut().poll_fill_buf(&mut cx)?, Poll::Ready(&[3][..]));
reader.as_mut().consume(1);
assert_eq!(reader.as_mut().poll_fill_buf(&mut cx)?, Poll::Pending);
assert_eq!(reader.as_mut().poll_fill_buf(&mut cx)?, Poll::Ready(&[][..]));

# Ok::<(), std::io::Error>(())
```
futures-test/src/io/read/mod.rs