- the mechanics of "backref" have been fully merged into the

- the mechanics of "backref" have been fully merged into the
finer grained "back_populates" system, and take place entirely
within the _generate_backref() method of RelationProperty.  This
makes the initialization procedure of RelationProperty
simpler and allows easier propagation of settings (such as from
subclasses of RelationProperty) into the reverse reference.
The internal BackRef() is gone and backref() returns a plain
tuple that is understood by RelationProperty.