Always add sort as a vertex to the graph in case it is a simple enum

[yiyunliu]

This patch should fix the issue mentioned in #28

The culprit is the build_graph function, which won't add enum sorts as vertices to the graph through the add_edge function as their constructors don't have any arguments (and thus have no edges).

When G.succ in build_signature gets called later, an exception would be thrown because the enum vertex is never inserted.