WIP: Nested named tuples

[cscherrer]

This PR is to allow nested named tuples, as first described in #126 (comment)

[cscherrer]

Hi @tkf , I have tests passing locally if I comment out

# if isdefined(StructArrays, :append!!)
#     append!!(xs::StructArrays.StructVector, ys) =
#         StructArrays.append!!(xs, ys)
# else
#     append!!(xs::StructArrays.StructVector, ys) =
#         StructArrays.grow_to_structarray!(xs, ys)
# end

in BangBang/src/structarrays.jl.

There's some type piracy here, and I think BangBang is a lightweight dependency anyway. But I guess things would need to be updated carefully to be sure there's a smooth transition. Any other thoughts on this?