simplifications in types

src/main/java/de/tla2b/types/AbstractHasFollowers.java

src/main/java/de/tla2b/types/EnumType.java

src/main/java/de/tla2b/types/StructOrFunctionType.java

src/main/java/de/tla2b/types/StructType.java

src/main/java/de/tla2b/types/TupleOrFunction.java

src/main/java/de/tla2b/types/TupleType.java