Helps with conversion of `Int` -> CartesianIndex.
This is now used within the indexing pipeline instead of jumping back
into `to_index`.
The `CartesianIndex` -> `Int` conversion is managed by composing a
`StrideIndex`, where the strides are computed using `size_to_strides`,
    instead of the internal memory representation.

Previously an array that needed a unique method for `known_size` also
needed a unique one for `known_size(::Type{A}, dim)`. Now `known_size`
is called and then indexed, requring only one new method.