Implement rendering for `FORC`s

[leonbohn]

Right now, there is an empty test display_forc that builds a FORC but then cannot render it to DOT as the corresponding code is missing. This is not really top priority, but we should still implement something here.

[leonbohn]

At the same time, we should remove the Show trait (as outlined here #68 ). Further, it could be sensible to remove the ability to display rendered things from release code, so that it can only be used for debugging purposes.