Refactor `util::pack_vector_fields` to use `array::ArrayViewVariant`

[odlomax]

This PR refactors util::pack_vector_fields to use std::variant<ArrayViews...> with std::visit, as opposed to nested switch statements to resolve the possible Value types and Rank values for different arrays.

I also found it beneficial to refactor array::make_view_variant such that it produces either a variant of const or a variant of non- const ArrayView types.

This PR needs #220 and #224 to be merged first.