Fix the star combinator
Fix reducer composition order
Better graph visualization

* Use pr-str for the parameter to Epsilon
* Run a placeholder value through the reducer function
Fix bfs type annotation
Add variadic cat, flat star, plus combinator

* Add 'merge-up' data to the Cat and Star combinators, indicating
  that the resulting parse tree should be merged with the parent's

* Add 'merge' metadata to parse trees which should behave as above. Look
  for it in the parse-tree constructor function. 

* Add a multiple-arity cat implementation which sets merge-up correctly

* Use the merging cat from Star

* Add a plus combinator to quickly create the right Cat / Star combination