The TypeGraph class should only be responsible for storing Type nodes.
Traversing the graph and tracking which nodes have been visited should
not be included there.
Passes now take a NodeTrackerHolder as an input parameter, which
provides access to a zeroed-out NodeTracker.
This removes Printer's legacy behaviour of generating an ID for each
node as it gets printed. This old method meant that if new nodes were
added to or removed from a graph, every ID after the new/removed node
would change.
Now IDs are stable so it is easier to follow specific nodes through
multiple transformation passes in CodeGen.
Use fully qualified names to determine if a class is really the child of
our type. It may be that it is the child of another type with an
identical name in another namespace.
