Investigate P-Token compilation artefacts, make graph generation work

[jberthold]

The P-Token is our guide to prioritising features in kmir development. Ongoing activity is to explore this code and the stable MIR that it compiles to.

• Refactor the graph generation options to enable producing both and to work with cargo
• The graph generation for P-token (using the --dot option) creates a dot file which cannot be cleanly processed by graphviz.

[jberthold]

For an analysis of the extracted Stable MIR see #485 , and apparently graphs could be generated that time.
Unclear whether graph generation was failing because of the host's graphviz installation or because of other problems.
I am closing this for now, we can re-open if need be.