- Standalone expressions in polymorphic_on

- Standalone expressions in polymorphic_on
propagate to single-table inheritance
subclasses so that they are used in the
WHERE /JOIN clause to limit rows to that
subclass as is the usual behavior.
- make sure implicit map to polymorphic_on expr
handles creating a label().  Use an explicit name
here as _sa_polymorphic_on makes more sense when
poking around in _props.