#1226 Various improvements to BddUtils.printAsDot/bddToDotRec.

• No end-user visible changes.
• Changes:
  • Renamed bddToVisitedIdx to bddToDotNodeIdx.
  • No more visited array. Just use bddToDotNodeIdx.
  • No more visitedNextFreeIdx. Just use bddToDotNodeIdx.size().
  • bddToDotRec documents responsibilities for freeing BDDs.
  • bddToDotRec no longer returns next free index, but the DOT graph node index of the given bdd.
  • bddToDotRec no longer has duplicate code for adding nodes.
  • Fixed BDD leaks.
  • Order of elements of the DOT graph has changed, affecting unit tests.

Closes #1226 (closed)