message: Fix to_string output of embedded v4 subnets
diff: @@ -87,6 +87,8 @@ int ipv6_subnet::compare(const ipv6_subnet& other) const noexcept {
  }
  std::string to_string(ipv6_subnet x) {
  + if (x.embeds_v4())
  + return to_string(x.embedded_v4());
  auto result = to_string(x.network_address());
  result += '/';
  result += std::to_string(x.prefix_length());
  