Allow introducing newtypes without a migration

[adamgundry]

If the type of a field is changed to a type that is equivalent after expanding newtypes, it would be nice to be able to omit the custom migration. At the moment a no-op custom migration has to be used. (Of course this assumes the new type does not have a smart constructor.)

This is somewhat similar to #34.

[adamgundry]

A similar issue: it would be nice if migration from an enum type to another enum type with the same (or a superset of the) constructors was permitted without a custom migration.