message: Fix deep_to_string for std::string
diff: @@ -247,6 +247,7 @@ public:
  && !std::is_pointer<T>::value
  && !is_inspectable<stringification_inspector, T>::value
  && !std::is_arithmetic<T>::value
  + && !std::is_convertible<T, string_view>::value
  && !has_to_string<T>::value>
  consume(T&) {
  result_ += "<unprintable>";
  