io: export Chain for users of AsyncReadExt::chain

[aatifsyed]

Motivation

I want to downcast a hyper::upgrade::Upgraded.
This also makes the code consistent with Take, and other combinators like those found in std::iter

Solution

[ADD-SP]

I want to downcast a hyper::upgrade::Upgraded.

Sorry, I don't understand your usecase, could you please elaborate your usecase?

[aatifsyed]

Sorry, I don't understand your usecase, could you please elaborate your usecase?

I'll rephrase :)

I want to downcast a trait object.
To downcast, I'd like to name the trait object.
I can't name a Chain. This PR makes that possible.

Also makes Chain more consistent with Take, which is already nameable.

[Darksonn]

We don't want to expose the future types, but Chain is like Take that is already exposed so I'm okay with this.