Remove 'adjacent?' from the base IGraph protocol
Remove 'update-edge' from the base IGraph protocol
Add naïve implementations of 'dissoc-node' and 'filter-nodes'
Better, reduce-based (instead of loop-based) implementation of 'filter-nodes'
Fold 'dissoc-node' into the base IGraph protocol
Resurrect 'adjacent?' as 'has-edge?'