make Parser.Symbol public to make implementation of EntryCallback and ExitCallback possible

Author: dieu

src/main/java/com/dashjoin/jsonata/Parser.java

@@ -92,7 +92,7 @@ public static<T> T clone(T object) {
         }
     }
 
-    class Symbol implements Cloneable {
+    public class Symbol implements Cloneable {
 
         //Symbol s;
         String id;