Support arbitrary AbstractArrays for refs

[Marlin-Na]

I wonder if there is plan to support arbitrary abstractarrays type for storing refs array? In certain cases, there might be interest from user to use a more efficient array storage - for example, using run length encoding to store repeated values, or using a lazy on-disk array type when representing a large dataset.

Alternatively, I would suggest to make AbstractCategoricalArray more easily extendable by defining several interface methods.

[nalimilan]

Nobody has requested this until now so this hasn't been considered. If you're interested in that you could try seeing whether it would be doable without making the code significantly more complex.