util: Add a TokioContext future

[LucioFranco]

We should add a TokioContext future that takes some future F and a "handle" to the runtime (in tokio 0.2 this will be Handle and in tokio 0.3 this will be Runtime). It should then implement the Future trait when F: Future and will wrap each call to poll of the inner future with Handle::enter. This gist has an example implementation called TokioIo.

This should also live behind some feature flag in util, possibly rt-core since that will line up with the features flags in the main crate.

[LucioFranco]

Another, possible addition here would be to include the ability to take some runtime "handle" and have a method on it that will wrap it and output a future that can be spawned on other executors but still use the tokio context.