Address a long standing bug in a Where.add optimization to discard
equal nodes that was surfaced by implementing equality for Lookup
instances in bbf141bcdc31f1324048af9233583a523ac54c94.
Thanks Shaheed Haque for the report.
Backport of b81c7562fc33f50166d5120138d6398dc42b13c3 from main
While Node class has a useful `__str__`, its `__repr__` is not that
useful. Added a `__repr__` that makes use of the current `__str__`.
This is especially useful since the more popular `Q` class inherits
`tree.Node`. Also created new tests that cover most of `Node` class
functionality.