Supporting these functions was a little tricky. For example, how should we
handle calling cycle/remove on the item that is currently focused? After
attempting to be clever, I decided to just set the value to nil and let the
consumer decide what is best for them. I can always support a more opinionated
version that fallsback to previous-index if previous-index is set. But until I
have a better idea of how I'm going to consume this, I think nil is the best
option.
Add predicate for determining if a cycle contains items.
Updated cycle/{new,from-list} to support setting current-index to nil when a
consumer calls it with an empty list.
