Simplify tuple flattening and destructuring

The `flatten_tuple` function was unnecessarily recursive. It can now
simply return the elements of the tuple directly. The VM's destructuring
logic has been updated to handle nested destructuring more efficiently.
A new integration test case for multi-level destructuring has been
added.

src/integration_test.rs

@@ -369,4 +369,25 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn test_multi_level_destructuring() {
+        let env = Environment::new();
+        let source = "(do
+          (def pipe (fn [conf]
+            (do
+               (def [str s] conf)
+               (def [f ss] s)
+               [\"Symbol:\" str \"field:\" f \"id:\" ss]
+            )
+           )
+          )
+          (pipe [\"btc\" [:close \"cls\"]]))";
+
+        let res = env.run_script(source).unwrap();
+        assert_eq!(
+            format!("{}", res),
+            "[\"Symbol:\" \"btc\" \"field:\" :close \"id:\" \"cls\"]"
+        );
+    }
 }