Column type for more than 256 categories

[BaxterEaves]

Currently Categorical column type only supports categories with up to 256 types. This is pretty limiting. There should be a Categorical type that supports usize::MAX categories.

[shauneccles]

This is very limiting for my use cases so would be very much appreciated as a feature.

[14lclark]

Has this been addressed yet? If not, what are the limiting factors for this?

More specifically, what traits/structs/functions need to be updated to get this to work? I imagine it's more than just changing all the u8 types whenever they're used related to ColType::Categorical, but I'm not sure where to start looking.