Support variant<void>, variant<T&>

This as a side effect will make it possible to remove the `result<void>` specialization, and will obviate the need for a `result<T&>` specialization.

`void` will need to be replaced in `variant_base` with `variant2::void_`, and `T&` - with either `std::reference_wrapper<T>` or `variant2::detail::refwrap<T>`.