New struct.Node base class for Link, List, and Struct.

Move various path handling pieces out of Struct and into a new Node
class. This way I can make List and Link tree aware so they can handle
their expansion themselves. The eventual goal is to get all of the
expansion related code out of the parser.

Also, name, _path, and root are now node_name, node_path, and tree_root.
This names are less ambiguous so I'm less likely to goof up and create an
attribute name conflict between Node and it's ch