Optimize for ZST value

[virusdefender]

A typical use case is using quick_cache as a HashSet with an eviction mechanism. In such scenarios, the value is usually a zero-sized type like PhantomData.

Since there's no need to update the stored values in these cases, concurrent management mechanisms such as PlaceHolder are unnecessary, thus reducing memory overhead.

[arthurprs]

Makes sense. I'd happily accept a PR for it as long as it's not costly to maintain.