Make `Response::bytes_stream` return a concrete type

[its-the-shrimp]

in my project I needed to store the resulting stream to avoid an allocation, but the fact that the method returns an impl Stream means that it wasn't possible to do so without boxing it

[seanmonstar]

Thanks for the PR! But I'm not going to expose a named type for this method.

You can store it by boxing it, as one option. But you said you don't want to allocate. So you could also just store the Response, or put it in a newtype and pull out a chunk in a Stream impl.