message: Fix set.map implementation
diff: % Extensions to Makam's standard library
  +
  %extend map.
  - keys : map K V -> set K -> prop.
  + keys : map Key Elem -> set Key -> prop.
  keys [] [].
  keys ((Key, _) :: Map) (Key :: Keys) :-
  keys Map Keys.
  %extend set.
  - map : (A -> prop) -> set A -> prop.
  - map P [].
  - map P (Elem :: Set) :-
  - P Elem,
  + map : (Key -> prop) -> set Key -> prop.
  + map Prop [].
  + map Prop (Elem :: Set) :-
  + Prop Elem,
  set.remove_if_member Set Elem Set',
  - map P Set.
  + map Prop Set'.
  %end.
  