message: implement proposed resolution of ericniebler/stl2#626
diff: #include <range/v3/range/traits.hpp>
  #include <range/v3/utility/static_const.hpp>
  #include <range/v3/view/all.hpp>
  +#include <range/v3/view/interface.hpp>
  #include <range/v3/view/single.hpp>
  #include <range/v3/view/view.hpp>
  @@ -336,7 +337,7 @@ namespace ranges
  if_then_t<ForwardRange<Base>, std::forward_iterator_tag,
  std::input_iterator_tag>;
  using iterator_category = std::input_iterator_tag;
  - struct value_type
  + struct value_type : view_interface<value_type>
  {
  private:
  split_outer_iterator i_ = split_outer_iterator();
  