Function concatenate

include/stringzilla/stringzilla.hpp:4570–4572

 */
template <typename first_type_, typename second_type_>
concatenation<first_type_, second_type_> concatenate(first_type_ &&first, second_type_ &&second) noexcept(false) {
    return {first, second};
}

/**
 *  @brief Concatenates two or more strings into a template expression.