c++ 17 one allocation strcat

strcat17.cpp

// fine for string_view or const char*. todo: use std::forward
// from Marco Magdy
template<typename.. Args>
auto strcat(Args... args)
{
  const auto total = (... + args.size());
  std::string s;
  s.reserve(total);
  (s += ... += args);
  return s;
}