Support sorting of `Vector{Union{T, Missing}}` for `RadixSort`

[xiaodaigh]

I think this can be done by adding and making some other accommodating changes

uint_mapping(Forward, missing) = 2^8sizeof(UInt)-1
uint_mapping(Reverse, missing) = UInt(0)

This is the approach taken in SortingLab.jl to support sorting of Vector{Union{Missing, T}} for radix-sort.

[nalimilan]

Interesting. Could you make a PR? Probably also worth supporting nothing.

[xiaodaigh]

BTW

sort([nothing, 2,3])

returns an error on Julia 1.3. So should not support nothing in solidarity