Module `Hash_set`

Ideas are based on Hash, however, Hash.add does not really optimize and has a bad semantics for Hash_set, This module fixes the semantics of add. remove is not optimized since it is not used too much

module Make: 
   (H: Stdlib.Hashtbl.HashedType) =>
  Hash_set_gen.S with type key = H.t;

A naive t implementation on top of hashtbl, the value is unit