Revise workspace/buffer to be ::UInt8[] instead of ::AbstractVector{eltype(input)}

[LilithHafner]

The workspace/buffer used in sorting is merely a place to put things. It need not have a fixed type. I believe the most natural way to represent a place to put things is a Vector{UInt8}, as suggested originally by cjdoris. The impact of this change is perhaps best understood by seeing how the "sort(x; workspace=w)" test set changes.

To use a workspace/buffer, one needs to designate some space workspace = UInt8[] and then give that space to multiple different sorting functions, each of which will temporarily assume ownership of that Vector, resize/reinterpret it as needed, and then return ownership back to the caller upon return. To quote cjdoris, "The caller can then preallocate an empty such vector and pass it in to repeated calls without needing to know how it will be used."

This is better than workspace::AbstractVector{eltype(input)} because it is more natural, extensible, and reusable (subjective) and because it allows for the workspace to be used both to store elements of the input array and to store other relevant collections with different element types like the count vector in radix sort (objective).

See discussion leading to this PR here and more recently here. This follows up on #45330.

[LilithHafner]

While I still think there is theoretical merit to this approach, I am giving up pursuant to this discussion.

The complexities are too large and the reward too small for me to do this without additional language support. I've folded separable parts of this PR into #45570.