[Grace-core] Encoding classes

[Kim Bruce]

> 
> 
> If our purpose is to type-check clients, then we need to put all of the client-visible information.  If our purpose is to type-check inheritors, then we need to put all of the information on which inheritors can depend.   If we can to use one type for both, then we need the join of those types.  Alternatively, we could have two types: one for inheritors and one for clients. 

Amusingly, that was the essence of my proposal for Modules with multiple interfaces :-)

Kim