Change behavior of eltype()

[nalimilan]

eltype(CategoricalString) should return Char for consistency with getindex. eltype(CategoricalValue) should not be defined, which means it returns Any (like any other type by default).

Cf. JuliaData/DataFrames.jl#1282.

Cc: @alyst

[alyst]

+1. The idea was that eltype() would be a synonym for leveltype(), but it's true that it would interfere with eltype() for container-like categorical values.
IIRC there are no other generic Base overrides for categorical values (except hash/isless/<, which make sense) that may conflict with the need to define Base methods for a specific categorical value.