Add oneshot::Sender::is_connected_to method

[canndrew]

I was gonna open an issue asking for this, but it was trivial to implement so I figured I'd just go straight to a PR.

This adds a method to oneshot::Sender to check whether it's connected to a given Receiver:

/// Tests to see whether this `Sender` is connected to the given `Receiver`. That is, whether
/// they were created by the same call to `channel`.
pub fn is_connected_to(&self, receiver: &Receiver<T>) -> bool {
    Arc::ptr_eq(&self.inner, &receiver.inner)
}

Does this seem like a good idea? It would be useful for me.

[taiki-e]

Thanks!