Support for relationship attributes in many_to_many accessors.

Support for relationship attributes in many_to_many accessors.
Merge new where condition, old where condition, cond in ResultSet::search_rs
Collapse cond in ResultSet::new_result so that it doesn't try to create columns with names like "-and".
Add a bunch of tests for the where condition things, which also happen to require the cond collapsing.
Create supporting classes and associated tables for these tests.