Add schema file support for FieldSets

[mattinsler]

Named field sets can be created by the generator rather than having to write them by hand.

data Person {
    field-set Personal {
        name,
        age
    }
    field-set Images {
        small_picture,
        large_picture,
        huge_picture
    }
    string name;
    int32 age;
    blob small_picture;
    blob large_picture;
    blob huge_picture;
}

This would just create

public abstract class Person {
    public static final FieldSet Images = FieldSet.with(
            Person.SmallPictureKey,
            Person.LargePictureKey,
            Person.HugePictureKey
    );
    ...
}

I'll leave the exclusions and array slicing syntax to be fleshed out later.