Methods like `convert(::Type{Any}, ::CategoricalValue)` triggers lots of invalidations.
Restricting the wrapped types to `AbstractString`, `AbstractChar` and `Number` alleviates
this problem without affecting usability too much.

One limit with this approach is that e.g. `convert(Union{String, T}, CategoricalValue{String})`
won't work anymore for any `T`, even though `convert(String, CategoricalValue{String})` will.
Though thanks to special casing, it will work for `T <: Missing` and `T <: Nothing`.

This also means that `CategoricalArray{Any}` is no longer supported. Adapt promotion rules
to ensure that mixing e.g. strings and integers gives `Union{String, Integer}`. Drop support
for `nothing`.

Also remove the custom `repr` method which was a legacy of the `CategoricalString` era.