[tokio-async-await] Stream support

[lnicola]

I was trying to port over some server code using hyper and I ran into this error:

error[E0277]: `(dyn futures::stream::Stream<Item=hyper::Chunk, Error=std::boxed::Box<(dyn std::error::Error + std::marker::Sync + std::marker::Send + 'static)>> + std::marker::Send + 'static)` cannot be shared between threads safely

After asking on Gitter, the issue might be that the compiler doesn't recognize that hyper::Body (which is, more or less, a Stream) can be Unpin.

[carllerche]

The problem is that Unpin is not implemented for trait objects.

The fix for this will take a few changes:

• Implement Unpin for Box, Rc, and Arc rust-lang/rust#53874
• impl Unpin for NotifyHandle with feature flag rust-lang/futures-rs#1239

[vorot93]

@carllerche Both seem to have been closed some time ago.

[carllerche]

I think this may no longer be an issue. I'm going to close it. If I am wrong, please reply as I misunderstood the problem.