This makes it easier to swap one fo the other without code breaking.
Since it’s strictly a removed API, I’ve opted to throw a deprecation
note in SS4 rather than throwing an InvalidArgumentException.
Fixes#2949
This prevents the map-to-ArrayData conversion from changing object
state and making the result of toArray() non-deterministic.
Fixes#2636. Other solutions were suggested on that ticket, but there is
no way of putting special code in for a SSViewer-specific iterator, and
having object state pre-emptively converted to ArrayData would
be an SS5-level change.
