feat: swap macro

mute-interpreter/src/env/standard.rs

@@ -19,6 +19,13 @@ pub(super) fn standard() -> HashMap<String, Value> {
                 (macro* (name args body) ~(define (,name (fn* ,args ,body))) )
             },
         ),
+        (
+            "swap",
+            inline! {
+                (macro* (a b) ~(define (,a ,b) (,b ,a)))
+            },
+        ),
+        // Quoting
         (
             "quote",
             inline! {