feat(*): new data model

[hishamhm]

Change the underlying data model from a graph of nodes where each link is of the form $(n_1, n_2)$ to a graph of nodes and ports, where each link is of the form $((n_1, p_1), (n_2, p_2))$.

This lets us handle multiple inputs and outputs of a node separately (e.g. headers and body of a call node).