Remove hash from module sig, fix equals

The hash functions in the Vertex- and Edge-label modules are unused, and
likely won't be used in the future, as this is a functional graph.

Equality of the internal Vertex- and Edge-label structures implemented
inconsistent hashing and equality functions. Fix that too.
Lift NETWORK sig out of MAKE sig

Need access to that sig in order to export it in other projects.
Don't package the library as a packet
next_hop returns the edge traversed

next_hop was throwing away essential information by just returning the
vertex. It must also return the port on that vertex. A simple way to
achieve this is to return the edge.
Replace endpoints with edge_src and edge_dst
Expose mutators in interface
Add remove_endpoint mutator
Maintain mapping between labels and handles