WIP: Make object implicitly share Array and Dict data

As a counter point to !121 (closed), this adds implicit data sharing to Object for Array and Dict data. Ensuring proper detaching of these values is currently done by Object only handing out const Array * instead of Array* and hiding mutable access to Array and Dict behind the helper methods provided by Object. This means that all code that is currently modifying possibly shared instances of these types must be changed to modify instance of Object instead, but is also allows us to find all places where mutable sharing is currently assumed. (Of course, the converse also holds, i.e. there might be places which already use the helper methods and additionally assume that modified Object contents is shared even between multiple Object instances.)

This is very much WIP as Dict does not implement implicit sharing yet and the relevant users have not been updated. But it should provide a useful basis for discussing the approach.