Generator: Add static list of fields in generated manager interfaces

[Pyknic]

Currently you need an instance of Manager to obtain a stream of fields, even though all the fields are static instances. We can add a static way of obtaining the stream by introducing a static immutable list of fields in the generated manager interface and use it in the generated Manager#fields()-method. This will change the generated code, but not change the API.

Example:

FilmManager.FIELDS.stream()
    .map(Field::identifier)
    .forEachOrdered(System.out::println)