Deprecate AbstractString methods for CategoricalString

[nalimilan]

This is the first step towards deprecating CategoricalString in favor of CategoricalValue{String}. (We can't just do that immediately since lots of string methods dispatch on AbstractString, from which CategoricalValue{String} cannot inherit.)

[bkamins]

I am OK with this change.

What is your final opinion about adding levelcode and levelvalue as we discussed on Slack? (I ask because I see we use get here now)

[nalimilan]

What is your final opinion about adding levelcode and levelvalue as we discussed on Slack? (I ask because I see we use get here now)

Yes, I think we should do that. I just wanted to make sure I like these names. :-)

@ablaom Do you have any opinion on merging CategoricalString and CategoricalValue{String}? I guess that would simplify the design a little bit for you, but I'd like to check that you don't rely on CategoricalString <: AbstractString.

[ablaom]

Thanks for consulting me. No, I don't rely on CategoricalString<:AbstractString and the proposed simplification makes sense to me.