This allows libraries that require access to reactor related types to
depend on this crate without having to depend on the entirety of Tokio.
For example, libraries that implement their custom I/O resource will
need to access `Registration` or `PollEvented`.
This patch is an intial implementation of the Tokio runtime. The Tokio
runtime provides an out of the box configuration for running I/O heavy
asynchronous applications.
As of now, the Tokio runtime is a combination of a work-stealing thread
pool as well as a background reactor to drive I/O resources.
This patch also includes tokio-executor, a hopefully short lived crate
that is based on the futures 0.2 executor RFC.
* Implement `Park` for `Reactor`
This enables the reactor to be used as the thread parker for executors.
This also adds an `Error` component to `Park`. With this change, a
`Reactor` and a `CurrentThread` can be combined to achieve the
capabilities of tokio-core.
