- base_columns on ColumnElement becomes a list; as usual, because columns in Compound...

- base_columns on ColumnElement becomes a list; as usual, because columns in CompoundSelects
may extend from more than one root column.
- keys_ok argument from corresponding_column() removed.  no more name-based matching of columns anywhere.
- DictDecorator is gone.  all row translators provided by orm.util.create_row_adapter().  Mapper
and contains_alias() cache the adapters on target mapper to avoid re-computation of adapters.
- create_row_adapter() accepts an "equivalent_columns" map as produced by Mapper, so that
row adapters can take join conditions into account (as usual again, to help with the CompoundSelects
produced by polymorphic_union).
- simplified TableSingleton to just provide lookup; moved all initialization into Table.
- the "properties" accessor on Mapper is removed; it now throws an informative
exception explaining the usage of mapper.get_property() and
mapper.iterate_properties