Change 'type identityref' Binding representation to normal objects

For 'identity' statements we currently generate a plain interface:

public interface Foo
    extends
    BaseIdentity
{
    public static final @NonNull QName QNAME = $YangModuleInfoImpl.qnameOf("foo");

}

without any instantiation. We then use these classes as values wherever we have an identityref. This leads to a rather not nice user experience, where users spell out Foo.class. Furthermore it is also a source of issues, as Class objects require different handling in Java source (see MDSAL-732) as well as runtime – instead of a plain Class.isInstance() checks an additional Class.isAssignableFrom() to ascertain type safety.

Update the design so that we generate a Foo.VALUE field, which implements the generated interface and also properly expose implementedInterface():

public interface Foo
    extends
    BaseIdentity
{
    public static final @NonNull QName QNAME = $YangModuleInfoImpl.qnameOf("foo");
    public static final @NonNull Foo VALUE = new Foo() {
        @Override
        Class<Foo> implementedInterface() {
            return Foo.class;
        }
    };

    @Override
    Class<? extends Foo> implementedInterface();
}

For identityref leaf/leaf-list members change the mapping from Class<? extends Foo> to plain Foo. Users will then use Foo.VALUE instead of Foo.class to identify instances.