Enable writing to ArrayHandleCast

Previously, ArrayHandleCast was considered a read-only array handle. However, it is trivial to reverse the cast (now that ArrayHandleTransform supports an inverse transform). So now you can write to a cast array (assuming the underlying array is writable).

One trivial consequence of this change is that you can no longer make a cast that cannot be reversed. For example, it was possible to cast a simple scalar to a Vec even though it is not possible to convert a Vec to a scalar value. This was of dubious correctness (it is more of a construction than a cast) and is easy to recreate with ArrayHandleTransform.