perf(list): use loop directly avoid rev

Author: illusory0x0

list/list.mbt

@@ -635,15 +635,21 @@ pub fn is_empty[A](self : T[A]) -> Bool {
 /// assert_eq!(b, @list.from_array([2, 4, 6]))
 /// ```
 pub fn unzip[A, B](self : T[(A, B)]) -> (T[A], T[B]) {
-  let mut xs = Empty
-  let mut ys = Empty
-  // implemented with loop to avoid stack overflow
-  loop self.rev() {
-    Empty => break (xs, ys)
+  match self {
+    Empty => (Empty, Empty)
     More((x, y), tail~) => {
-      xs = More(x, tail=xs)
-      ys = More(y, tail=ys)
-      continue tail
+      let xs = More(x, tail=Empty)
+      let ys = More(y, tail=Empty)
+      loop tail, xs, ys {
+        Empty, _, _ => ()
+        More((x, y), tail~), More(_) as xptr, More(_) as yptr => {
+          xptr.tail = More(x, tail=Empty)
+          yptr.tail = More(y, tail=Empty)
+          continue tail, xptr.tail, yptr.tail
+        }
+        _, _, _ => abort("unreachable")
+      }
+      (xs, ys)
     }
   }
 }