selector bytes.jl

"""
    Base.selectorbytes(A::Array{T, N}) -> Array{UInt8, N}

For an Array with `isbits` Union elements, return the "selector bytes" that indicate the
index of the type for each array element, i.e. the type of `A[1]` is
`Base.uniontypes(eltype(A))[Base.selectorbytes(A)[1] + 1]`.
**NOTE**: The actual array selector bytes are returned, meaning if individual elements
are modified, the original array will reflect those changes. Setting selector bytes to
invalid or out-of-bounds type indexes may corrupt the original array.
"""
function selectorbytes(a::Array{T, N}) where {T, N}
    isbitsunion(T) || return UInt8[]
    return unsafe_wrap(Array{UInt8, N}, convert(Ptr{UInt8}, pointer(a)) + length(a) * elsize(a), size(a))
end