Pack/unpack should use getproperty instead of getfield

[mauro3]

Also fieldnames should be replaced with propertynames, presumably with private=false, see https://docs.julialang.org/en/latest/base/base/#Base.propertynames.

x-ref: JuliaLang/julia#24960

[mauro3]

I started on this in https://github.com/mauro3/Parameters.jl/tree/m3/properties

If I don't get side-tracked, I plan to finish this at the JuliaCon hackathon.