remove a few warnings

src/main/java/carpet/script/exception/ExitStatement.java

@@ -5,7 +5,7 @@ import carpet.script.value.Value;
 /* Exception thrown to terminate execution mid expression (aka return statement) */
 public class ExitStatement extends RuntimeException
 {
-    public Value retval;
+    public final Value retval;
     public ExitStatement(Value value)
     {
         retval = value;