HashSet support

[segeljakt]

Subject of the feature

It would be nice to have a halfbrown::HashSet<K> that works well for a small number of keys.

It could be implemented as:

struct HashSet<K>(halfbrown::HashMap<K, ()>);

[Licenser]

That sounds cool :D I think it's not too complicated I'll put it on the TODO list but I don't know when I'll get to that, but if you want to make a PR I'll happily merge it :)