Implement promises/channels engine

[joshprk]

Blocking tasks such as network requests or file reads can block the runtime loop, so we need a way to create promises and then retrieve the results later.

[joshprk]

Instead of straight promises, this will likely be implemented in 0.2.0 as a named channel:

#[component]
fn App(c: &mut Component) -> Inner {
    let channel = c.channel();

    channel.listen(|event| {
      // normal listener, and it still follows deterministic ordering of listener execution
    });

    c.listen::<MouseClick>(|event| 
        channel.spawn(|tx| ...)
    );

    Inner::default()
}