Reqwest: Update to std::future::Future

[encombhat]

Tokio is moving to std::future::Future. It's cool if reqwest's async client can also move to std::future::Future.

[seanmonstar]

Yes, this would be great! I'm personally busy with updating the dependencies of reqwest, but they may be ready enough for this to be started, if anyone else wants to start!

[jakubadamw]

I have done some semi-trivial changes in this direction in #573. I'll see if I can continue tomorrow or it could be just as easily picked up someone, if I won't manage.

[danieleades]

apologies for all the spamming.
Have opened a new pull request for this work (#592)
Unlike #573, I think it might be a cleaner approach to continue depending on the same version of hyper, tokio etc and simply use the futures compat layer to begin working with futures0.3. When the new versions of those libraries fully stabilise, we can just remove the compat layer.

Definitely looking for a little assistance on this one, if anyone can spare the brain cells

[jnicholls]

@danieleades Hmm, maybe. However, tokio 0.2 alpha is out now. When using tokio 0.2, the current reqwest::async::Client fails to spawn a task, because no thread-local runtime is found. I think it would be best to move in sync with tokio and hyper by extension.

[seanmonstar]

With the hyper alpha out, we can pin to those! I'll be focusing my attention on this issue now.

[jnicholls]

@seanmonstar Yep, I am actually 50% of the way through swapping out reqwest for hyper + hyper-tls + tokio-tls + native_tls directly. Haha. If you finish pinning to the alphas today, it is going to be bittersweet for me 😆