split backend into a file for each contained aspect

src/main/jastadd/backend/API.jadd

src/main/jastadd/backend/AbstractGrammar.jadd

src/main/jastadd/backend/Backend.jadd

src/main/jastadd/backend/BidirectionalAPI.jadd

src/main/jastadd/backend/DirectedAPI.jadd

src/main/jastadd/backend/LowerBoundCheck.jadd

src/main/jastadd/backend/NameResolution.jadd

src/main/jastadd/backend/PrettyPrinting.jadd

src/main/jastadd/backend/Serializer.jadd