Support SELF keyword in Podlang parser (#368)

* Support SELF keyword in Podlang parser

* Add pretty-printing for SELF

src/lang/pretty_print.rs

@@ -507,6 +507,16 @@ mod tests {
         assert_round_trip(&input);
     }
 
+    #[test]
+    fn test_round_trip_self() {
+        let input = r#"
+            self_test(Pod) = AND(
+                Equal(?Pod["self"], SELF)
+            )
+        "#;
+        assert_round_trip(input);
+    }
+
     #[test]
     fn test_pretty_print_demonstration() {
         let input = r#"