handle stack with abstract eltype

[adienes]

fixes #56771

probably not perfectly universally robust, but I bet this catches 95%+ of encountered instances.

[mcabbott]

I think the two problems with this approach are (1) it calls first, while everything else in stack is careful to never run the iterator more than once, and (2) it fails for other abstract types:

julia> it = Iterators.product(1:2, 3:5);

julia> stack(Iterators.ProductIterator[it for _ in 1:4]; dims=2) |> size
(2, 3, 4)