Requiring descriptors to have matching finality #61
Description
We could optionally require a descriptor type to be final if and only if its described type is final. With the fixed validation rules from #58, it's impossible to define a subtype of either an open described or descriptor type if the other is final, so in practice one being final makes the other final. Requiring them both to be explicitly final in that case makes it slightly simpler for the engine to optimize casts, etc.