Rewrite buildForest in terms of graphs (and strongly-connected

components)

This commit rewrites the logic behind `buildForest` to build first a
graph, discovering loops in there (via the strongly-connected components
machinery) and finally converting the DAG (freshly-made so) into a
Forest.

test/Test/Offline/Ngrams.hs

@@ -315,8 +315,8 @@ testLoopBreaker_02 =
   let t1 = Map.fromList [ ( "foo", mkMapTerm "foo" & ne_children .~ mSetFromList ["bar"])
                         , ( "bar", mkMapTerm "bar" & ne_children .~ mSetFromList ["foo"])
                         ]
-  in (buildForestOrBreakLoop t1) `compareForestVisually` [r|
-         foo
+  in (pruneForest $ buildForestOrBreakLoop t1) `compareForestVisually` [r|
+         bar
          |
-         `- bar
+         `- foo
 |]