Add a template type alias `scratch::nth_t<I, Ts...>` for the nth element of a pack.
Add a template type alias `scratch::nth_t<I, Ts...>` for the nth element of a pack.

The equivalent for (perfectly forwarded) values is basically

    std::get<I>(std::forward_as_tuple(std::forward<Vs>(vs)...))

but since I haven't implemented `std::tuple` yet, I'm passing on that one.

Quuxplusone/from-scratch,Quuxplusone/from-scratch,Quuxplusone/from-scratch