Add compile-time validation for effectively always cancelling listeners

[PaintNinja]

Suggests changing code that always returns true inside a possibly cancelling listener into a void return always cancelling listener, as the latter allows EventBus to know at listener registration time and can perform additional optimisations based on that knowledge.

Closes #110.

For example:

// Before
@SubscribeEvent
static boolean alwaysCancellingListener(ExampleCancellableEvent event) {
    return true;
}

// After suggested change:
@SubscribeEvent(alwaysCancelling = true)
static void alwaysCancellingListener(ExampleCancellableEvent event) {}