Keyed Set: a hashbrown-based HashSet that indexes based on projections of its elements.

Ever wanted a HashMap<K, V>, but where V actually contains K (or at least can be projected to it)? Well this is it.

The easiest way to define a projection is through a closure that you pass at construction, but you may also define your own key extractors as ZSTs that implement Default to gain a Default constructor for your Keyed Sets.