Migrating models along theory maps requires Julia types

When we migrate a model contravariantly along a theory map, we may need to create new types for the type constructors in the domain theory.

This issue is thinking about how to do this, in connection with #73.

One way to do this is to make these some sort of "anonymous structs", i.e. a NamedTuple or wrapper of such. This is not ideal because we like to have good names for types.

Another way to do this is to declare the structs as part of the invocation of the theory map to migrate the model. This is perhaps the most attractive option, because we can find out the types assigned to the type constructors, and avoid needing type parameters for the structs declared.

A third option is to declare the structs ahead of time and then pass them in. This is useful for "on-the-fly" model migration. I.e., we can declare a "model migration struct" from a theory map, which has a field which corresponds to the original model.