Cleanup/document, mainly grammars

src/main/scala/leon/grammars/Aspect.scala

src/main/scala/leon/grammars/Closures.scala

src/main/scala/leon/grammars/Empty.scala

src/main/scala/leon/grammars/ExpressionGrammar.scala

src/main/scala/leon/grammars/Grammars.scala

src/main/scala/leon/grammars/Label.scala

src/main/scala/leon/grammars/NonTerminal.scala

src/main/scala/leon/grammars/SimpleExpressionGrammar.scala

src/main/scala/leon/grammars/aspects/ExtraTerminals.scala

src/main/scala/leon/grammars/aspects/PersistentAspect.scala

src/main/scala/leon/grammars/aspects/SimilarTo.scala

src/main/scala/leon/grammars/aspects/Sized.scala

src/main/scala/leon/grammars/aspects/Tagged.scala

src/main/scala/leon/purescala/Types.scala

src/main/scala/leon/synthesis/graph/Graph.scala