Refactor dotfile generation

[quchen]

c59eadc94ca7d10dbfad467ac8278ec010f4780e introduces candidate refactorings to port dotfile generation to Text instead of String, making better use of Foldable, and combining node targets à la A -> {X,Y,Z}.

Using a small script (parser + prettyprinter), the 3cluster.dot file was semi-manually converted to the smaller dot-file representation in 65ec25887a23d94680e6a917899ee36b9936d96b already.

These changes should be merged into the development branch as soon as it is stable enough again to sustain a network worth plotting, making testing possible.