The tests for creating new instances or copying instances of Node and
its subclasses didn't fully capture the behaviour of the implementation,
particularly around whether the `children` list or is contents were the
same as the source.
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.
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.
