Provide a mix-in to class based models to allow deep copies

[mdbergmann]

cl-gserver currently only sets (setf) the value of state in (cons back-msg state) to the actor state. It does not make a deep copy to maintain immutable environments. This is up to the user.
However, it could provide at least a model mix-in that defines a generic function make-copy which implementer scan implement to make a deep copy of a component that cl-gserver could automatically use when setfing the state.