Add recursive type checks for Key values which themselves are CatDict instances.

[lzkelley]

Currently, if the value for a key in an Entry or CatDict is itself a CatDict (instead of a primitive; see for example), then a string literal is used as the key instead of a Key instance. This is because the Key class doesn't know how to handle a value which is not one of the (current) built-ins {STRING, NUMERIC, BOOLEAN...}.

Add a way to use Key instances to handle composite class values. Probably the best thing to do is to have the corresponding KeyCollection do the type checking...