Implement ContractEdge

[james-d-mitchell]

For a digraph, node, and node.

[james-d-mitchell]

@baydrea @Joseph-Edwards and HJ when you have a github handle!

[james-d-mitchell]

@hrj4 too

[mtorpey]

@saffronmciver Here's some info on the specifics of edge contraction: https://en.wikipedia.org/wiki/Edge_contraction

There are still questions here:

• Do we allow multiple edges or remove them?
• How do we handle mutable/immutable digraphs?
• Do we preserve the contracted edge as a loop?

but we can talk about them later, and this should get you started.