JSON serialization of CategoricalValue{T, R} is broken

[alyst]

Now CategoricalValue is not a <: AbstractString, so JSON.jl serializes it using CompositeTypeWrapper and goes into recursion due to circular pool -> value -> pool references.

julia> using CategoricalArrays, JSON

julia> x = categorical([1, 2, 3]);

julia> JSON.json(x[1])
ERROR: StackOverflowError:
Stacktrace:
 [1] Type at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:18 [inlined]
 [2] lower(::CategoricalArrays.CategoricalPool{Int64,UInt32,CategoricalArrays.CategoricalValue{Int64,UInt32}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:40
 [3] show_pair(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::Symbol, ::CategoricalArrays.CategoricalPool{Int64,UInt32,CategoricalArrays.CategoricalValue{Int64,UInt32}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:236
 [4] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::JSON.Writer.CompositeTypeWrapper{CategoricalArrays.CategoricalValue{Int64,UInt32}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:294
 [5] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::Array{CategoricalArrays.CategoricalValue{Int64,UInt32},1}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:302
 [6] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::JSON.Writer.CompositeTypeWrapper{CategoricalArrays.CategoricalPool{Int64,UInt32,CategoricalArrays.CategoricalValue{Int64,UInt32}}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:294
 [7] show_pair(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::Symbol, ::CategoricalArrays.CategoricalPool{Int64,UInt32,CategoricalArrays.CategoricalValue{Int64,UInt32}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:236
 [8] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::JSON.Writer.CompositeTypeWrapper{CategoricalArrays.CategoricalValue{Int64,UInt32}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:294
 [9] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::Array{CategoricalArrays.CategoricalValue{Int64,UInt32},1}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:302
 [10] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::JSON.Writer.CompositeTypeWrapper{CategoricalArrays.CategoricalPool{Int64,UInt32,CategoricalArrays.CategoricalValue{Int64,UInt32}}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:294
 [11] show_pair(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::Symbol, ::CategoricalArrays.CategoricalPool{Int64,UInt32,CategoricalArrays.CategoricalValue{Int64,UInt32}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:236
 [12] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::JSON.Writer.CompositeTypeWrapper{CategoricalArrays.CategoricalValue{Int64,UInt32}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:294
 [13] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::Array{CategoricalArrays.CategoricalValue{Int64,UInt32},1}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:302
 [14] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::JSON.Writer.CompositeTypeWrapper{CategoricalArrays.CategoricalPool{Int64,UInt32,CategoricalArrays.CategoricalValue{Int64,UInt32}}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:294
 [15] show_pair(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::Symbol, ::CategoricalArrays.CategoricalPool{Int64,UInt32,CategoricalArrays.CategoricalValue{Int64,UInt32}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:236
 [16] show_json(::JSON.Writer.CompactContext{Base.AbstractIOBuffer{Array{UInt8,1}}}, ::JSON.Serializations.StandardSerialization, ::JSON.Writer.CompositeTypeWrapper{CategoricalArrays.CategoricalValue{Int64,UInt32}}) at /home/astukalov/.julia/v0.6/JSON/src/Writer.jl:294

It could be easily fixed by implementing JSON.lower(x::CategoricalValue) method, but that would require introducing CategoricalArrays -> JSON dependency.
The more generic fix would be to implement support for circular references in JSON, but that would still print CategoricalValue incorrectly (probably we just need to show the stored value, i.e. get(x::CategoricalValue)).