Bump to Elm 0.19

Because Basics.toString is no longer available in optimized builds, we have to be a little creative. String escaping in DOT export in particular incurred a dependency on elm/json.

Fixed DOT exporting for empty graphs

• Removed semicolons from generated Graph.DOT.output

Revamped DOT exporting

• Renamed Graph.GraphViz to Graph.DOT, because DOT isn't specific to GraphViz
• Changed the interface of Graph.DOT.output* to match that of Graph.TGF with explicit node and edge label projections
• The output by Graph.DOT references nodes by their id instead of some text label, so that labels don't have to be unique or existant

Support for export to Trivial Graph Format

4.1.0

release version 4.1.0

Proper handling of acyclic graphs

Previously, results of topologicalSort and heightLevels was not really meaningful for cyclic graphs.

This release introduces checkAcyclic which returns an AcyclicGraph in the succeeding case, witnessing the fact that the graph is acyclic.
topologicalSort and heightLevels now take an AcyclicGraph as argument, and stronglyConnectedComponents returns a Result, being a more elaborate version of checkAcyclic.

GraphViz + elm-format validation

This release added the new Graph.GraphViz module (thanks, @jschomay!) for dumping a graph to a dot file.

Also, I began using [email protected] for formatting and added a validation step to the CI.