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 item of the stream.

# Examples

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

use futures::task::Poll;
use futures::stream::{self, Stream};
use futures_test::task::noop_context;
use futures_test::stream::StreamTestExt;

let stream = stream::iter(vec![1, 2]).interleave_pending();
let mut stream = pin!(stream);

let mut cx = noop_context();

assert_eq!(stream.as_mut().poll_next(&mut cx), Poll::Pending);
assert_eq!(stream.as_mut().poll_next(&mut cx), Poll::Ready(Some(1)));
assert_eq!(stream.as_mut().poll_next(&mut cx), Poll::Pending);
assert_eq!(stream.as_mut().poll_next(&mut cx), Poll::Ready(Some(2)));
assert_eq!(stream.as_mut().poll_next(&mut cx), Poll::Pending);
assert_eq!(stream.as_mut().poll_next(&mut cx), Poll::Ready(None));
```
futures-test/src/stream/mod.rs