Update to std::future::Future

[seanmonstar]

This is a tracking issue for updating to use the stabilized std::future::Future (in Rust 1.36).

[seanmonstar]

We can start this on an std-future branch now. Any middleware wanting to use things from tokio can target it's std-future branch.

The first step is just figuring out Service::poll_ready, as that likely needs to be Pin now. However, Service::call usually wouldn't need to be Pin, as it doesn't poll a future but return one. The problem is that if a Service isn't Unpin, once you've pinned it for poll_ready, you can't get a &mut self for call.

This leads to wondering if the trait should be split somehow into Service and Ready...

[seanmonstar]

Largely done, see #367 for status of each crates release.