StrJoin looks not optimized for tuples/multiple arguments

[MBkkt]

https://godbolt.org/z/YsYq1q14P

template <typename... Args>
std::string StrJoin2Args(std::string_view separator, const Args&... args) {
    return absl::StrJoin({static_cast<const absl::AlphaNum&>(args).Piece()...},
                         separator);
}

template <typename Tuple, size_t... I>
std::string StrJoin2UnpackImpl(std::string_view separator, Tuple&& t,
                               std::index_sequence<I...>) {
    return StrJoin2Args(separator, std::get<I>(t)...);
}

template <typename... T>
std::string StrJoin2(const std::tuple<T...>& value, std::string_view separator) {
    return StrJoin2UnpackImpl(separator, value,
                              std::make_index_sequence<sizeof...(T)>{});
}

Such implementation will resize string only once, so will do single allocation instead of O(n)

[derekmauro]

I've merged your PR to optimize this case #1672. Thanks!