Restrictions on key objects

To support base data types, and for performance, key objects for all kinds of maps are owned (or pointed to) using naked pointers, and are copied using copy constructors.

So:

• Since key objects derived from Listable aren't owned using the Wrapper idiom, they cannot be referred to by a Ref collection.

• Not only do key objects not need to be derived from Listable, if possible, they shouldn't be. For example, an owned string key should be a String, not a StringElem.