(async) Use the futures 0.3 - compatible versions of hyper and tokio

[jebrosen]

hyper 0.13 and tokio 0.2 are slated to use futures 0.3. We should use those, ideally by the first release of async-capable Rocket. This would allow us to remove our own dependencies on the futures 0.1 ecosystem and the futures 0.1-0.3 compatibility layer.

The following issues make that inconvenient today:

• These are currently the master branches of those projects, planned for hyper 0.13 and tokio 0.2. The port to futures 0.3 is still fresh, and we don't want to point to hyper@master until it settles a bit.
• We need hyper::body::Sender to implement the new Sink trait, or find a different approach for that section of code. Using a different approach.
• ~~ hyper's requirements on executors - see also (async) Support launching on a custom runtime #1068 ~~ hyper requires a tokio runtime as its executor, so this is resolved
• Keep in mind that this update should only affect internal APIs. If the futures 0.1 ecosystem has already "leaked" into rocket's public APIs, we have done something wrong!

[jebrosen]

I've made a proof of concept / WIP: https://github.com/SergioBenitez/Rocket/compare/async...jebrosen:async-futures03-hyper?expand=1

This makes use of https://github.com/Nemo157/futures-tokio-compat because tokio's AsyncRead is different from the one in futures-preview. Given this and other incompleteness (see tokio-rs/tokio#1201), I would still like to wait a bit longer to resolve this.

[jhpratt]

Tokio's 0.2 alpha has been released, which includes support for std::future::Futures.

[jebrosen]

Unfortunately hyper is completely broken for a few days due to some changes in tokio. It will maybe be fixed by hyperium/hyper#1890.

[jebrosen]

I have updated the async branch to use tokio 0.2.0-alpha.2 and an arbitrary compatible version of hyper.

As the commit message explains, tokio-rs/tokio#1356 causes some tests to fail. My own other mistake is also causing some of the same tests to fail, and I would like to wait for the upstream tokio issue to be resolved before trying too hard to fix those.