add support for unsigned long long

Want to be able to format the following without any casting

std::cout << fmt::str(fmt::Format("{:x}") << 0xAEull) << std::endl;
std::cout << fmt::str(fmt::Format("{:x}") << 0xAEul) << std::endl;
std::cout << fmt::str(fmt::Format("{:x}") << uint64_t(0xae)) <<
std::endl;