message: elle.meta: Add size.
diff: @@ -36,6 +36,9 @@ namespace elle
  template <template <typename> class P>
  using filter = typename filter_helper<List<>, List<Elts...>, P>::type;
  + /// Size.
  + static const int size = sizeof...(Elts);
  +
  /// The position of T in List
  template <typename T>
  struct
  