`UnknownArrayHandle` has special behavior when putting in or getting out
an `ArrayHandleMultiplexer` or an `ArrayHandleCast`. When putting in
either of these, `UnknownArrayHandle` gets the actual array stored in
the multiplexer and cast so that you can later retrieve the base array.
Likewise, when you get the array out with `AsArrayHandle`, you can give
it an `ArrayHandleCast` or `ArrayHandleMultiplexer`, and you will get
the base array placed inside of it.