message: C++: add String::str() function
  for convenience
diff: @@ -218,6 +218,7 @@ class String : public Sized {
  size_t length() const { return size(); }
  const char *c_str() const { return reinterpret_cast<const char *>(data_); }
  + std::string str() const { return std::string(c_str(), length()); }
  static String EmptyString() {
  static const uint8_t empty_string[] = { 0/*len*/, 0/*terminator*/ };
  