Node._new_instance() was added in
6dd2b5468f to work around Q.__init__()
having an incompatible signature with Node.__init__().
It was intended as a hook that could be overridden if subclasses needed
to change the behaviour of instantiation of their specialised form of
Node. In practice this doesn't ever seem to have been used for this
purpose and there are very few calls to Node._new_instance() with other
code, e.g. Node.__deepcopy__() calling Node and overriding __class__ as
required.
Rename this to Node.create() to make it a more "official" piece of
private API that we can use to simplify a lot of other areas internally.
The docstring and nearby comment have been reworded to read more
clearly.
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 bbf141bcdc.
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.
