This patch brings the merge function of CCPersistentHashtbl
in line with the merge functions of other maps
(Map, BatMap from batteries). In particular, its signature
changes from the restrictive
  merge: (key -> 'a option -> 'a option -> 'a option) ->
    'a t -> 'a t -> 'a t
to a more general
  merge: (key -> 'a option -> 'b option -> 'c option) ->
    'a t -> 'b t -> 'c t

As per maintainer request.