more lenient as to the table
being updated.  Plain Table objects are better
supported now, and additional a joined-inheritance
subclass may be used with update(); the subclass
table will be the target of the update,
and if the parent table is referenced in the
WHERE clause, the compiler will call upon
UPDATE..FROM syntax as allowed by the dialect
to satisfy the WHERE clause.  Target columns
must still be in the target table i.e.
does not support MySQL's multi-table update
feature (even though this is in Core).
PG's DELETE..USING is also not available
in Core yet.

that change object state as these aren't intended for public
use.

- break out key mechanics of loading objects
into new "orm.loading" module, removing implementation
details from both mapper.py and query.py. is analogous
to persistence.py
- some other cleanup and old cruft removal