fields in type objects should be private properties

[ericphanson]

julia> struct A{T}
       x::T
       end

julia> propertynames(A)
(:var, :body)

I think one should have to call propertynames(A, true) to get var and body.

Why does this matter? For me, I accidentally passed A instead of A() into something expecting a Tables.jl-compatible row, and since that generic code uses propertynames to find the "columns" of the "row", it grabbed var and body (which then Arrow tried to serialize, resulting in 200+ line unreadable stacktraces).

If this is breaking, I think we should still do it for 2.0.

[JeffBezanson]

Seems like a reasonable suggestion. Let's think about how breaking this might be; possibly not very.

[JeffBezanson]

Triage thinks we should do this and run PkgEval. It will probably be ok.