We need a Gcntree_node_data type

[nlevenson]

We often want to compare Gcntree_nodes to determine what part of a standard schema they represent. To do this, we probably need a Gcapp_node_data type which is parameterized according to the standard schema it's associated with -- i.e., there's a Gcapp_node_data.type property which stores the hash of the part of the schema it's associated with, such that we can easily determine how a Gcntree_node corresponds to the standard schema which defines the standard it was derived from.

We currently use brittle and bad methods to make these kinds of comparisons, like checking typeof node.data. These brittle methods typically make some gross assumptions about the design of a standard schema.