Expose task::Context to lazy

[srijs]

I'm currently working on moving the hyper 0.12.x branch over to the futures 0.2 alpha release.

As part of this, I'm starting to notice a few places where I would like to get hold of task::Context without needing to write a special future for it.

Here is an example:

fut.and_then(move |res| {
    // how do I get access to `cx`?
    if let Ok(Async::Pending) = pooled.tx.poll_ready(cx) {
        // do stuff
    }

    Ok(Async::Ready(res))
});

I tried using poll_fn, since that gives me the task::Context as an argument, but since it has a FnMut it doesn't work for most of cases I've tried.

Therefore I would like to propose this change, introducing task::Context as an argument to the closure passed in to lazy, similar to how it works for poll_fn.

Let me know what you think!

[cramertj]

Thanks for the PR! This makes sense to me.