Graphviz

[james-d-mitchell]

This is a reworking of #639, which goes a bit further than #639 in removing the old stuff for displays. This PR doesn't introduce any breaking changes except that we now require the graphviz package. So this PR shouldn't be merged until there is a release of `graphviz'.

[james-d-mitchell]

TODO:

• [x] doc
• [x] tests for code coverage

[james-d-mitchell]

The CI will likely fail here until:

https://github.com/digraphs/graphviz/pull/21

is merged.