Better api generation

Things to consider:

• breaking changes (removed imports), so up minor version, because we are still not in 1.0

src/main/jastadd/backend/AbstractGrammar.jadd

@@ -3,6 +3,7 @@ aspect BackendAbstractGrammar {
   public static String ASTNode.listInterface = "java.util.List";
   public static String ASTNode.listClass = "java.util.ArrayList";
   public static String ASTNode.jastAddListType = "List";
+  public static String ASTNode.jastAddOptType = "Opt";
 
   public static boolean ASTNode.resolverHelper = false;
   public static boolean ASTNode.serializer = false;