Generate value utility methods interface

[alicederyn]

Rather than requiring users type abstract methods for useful value utilities like .toBuilder() or .streamX(), generate an interface they can implement, with defaults so any other subclass is not negatively affected when we add new methods.

[alicederyn]

This one seems weightier than I'd anticipated, on reflection: the user type won't appear fully valid until after we've dropped out the code, and we can't reliably tell whether the missing type is the one we're meant to create, or one we're meant to parse for getters, meaning we need to start doing two-stage generation. See also #11.