Consider merging the functionality of the node.Node proxy into the concrete ast.Node

[lelit]

The proxy has been there since the beginning, because in v1 it dealt with the JSON structure returned by libpg_query's parse_sql() function. Now that there is a set of concrete ast classes, maybe it would make sense to move the hierarchy tracking data (parent_node and parent_attribute) into the concrete nodes. Will think about this for v4.