Supporting union bits types

[ViralBShah]

I tried to see what it might take to run DataFrames.jl on GPUs as discussed in JuliaData/DataFrames.jl#2713

Working with the neuro dataset from RDatasets, I could not convert a vector with missing data into a CuArray:

julia> cu(neuro.V1)
ERROR: CuArray does not yet support union bits types
Stacktrace:
  [1] error(s::String)
    @ Base ./error.jl:33
  [2] CuArray{Union{Missing, Float64}, 1}(#unused#::UndefInitializer, dims::Tuple{Int64})
    @ CUDA ~/.julia/packages/CUDA/Zmd60/src/array.jl:18
  [3] (CuArray{Union{Missing, Float64}, N} where N)(#unused#::UndefInitializer, dims::Tuple{Int64})

[ViralBShah]

cc @quinnj

[maleadt]

There is already an issue, #103, so I'll add your use-case there.