When the pk was a relation field, qs.filter(pk__in=qs) didn't work.
In addition, fixed Restaurant.objects.filter(place=restaurant_instance),
where place is an OneToOneField and the primary key of Restaurant.
A big thank you to Josh for review and to Tim for review and cosmetic
edits.
Thanks to Beauhurst for commissioning the work on this ticket.
Field.rel is now deprecated. Rel objects have now also remote_field
attribute. This means that self == self.remote_field.remote_field.
In addition, made the Rel objects a bit more like Field objects. Still,
marked ManyToManyFields as null=True.
Previously related fields didn't implement get_lookup, instead
related fields were treated specially. This commit removed some of
the special handling. In particular, related fields return Lookup
instances now, too.
Other notable changes in this commit is removal of support for
annotations in names_to_path().