Explicit representation of copies and merges in wiring diagrams

[epatters]

In wiring diagrams, copies and merges are represented implicitly by multiple wires, while deletions and creations are represented by the absence of wires. This implicit representation is usually the most convenient (effectively giving normal forms for diagonals and codiagonals) but sometimes we want to make the morphisms explicit, representing them by junction nodes as in string diagrams.

Write a function to insert the junction nodes where appropriate. A good first step towards implementing both #42 and #43.

[epatters]

Done, except that serialization is not supported for junction nodes. For now, just serialize using the default, implicit representation.