What do you think about this accepting `Notification` instead of `Action0`? The reason is that we now have 2 wrappers ... the `Notification` and `Action0` around a type `T` and 2 object allocations for each `onNext`.

The use of `Action0` is definitely more generic, but as we've seen by your `CompositeSubscription` changes, we're at the point where we're moving away from generic to achieve performance and memory gains, and this class will be involved in very high throughput scenarios.