module Make: 
Functor building an implementation of a weak hash table
Propose the same interface as usual hash table. However since
      the bindings are weak, even if mem h k is true, a subsequent
      find h k may raise Not_found because the garbage collector
      can run between the two.
      Moreover, the table shouldn't be modified during a call to iter.
      Use filter_map_inplace in this case.
include Hashtbl.S
val clean : 'a t -> unit
remove all dead bindings. Done automatically during automatic resizing.
val stats_alive : 'a t -> Hashtbl.statistics