make @list.unzip a method (#1444)

* make unzip a method

* update mbti

---------

Co-authored-by: Bao Zhiyuan <baozhiyuan@idea.edu.cn>

immut/list/list.mbt

@@ -643,8 +643,8 @@ pub fn is_empty[A](self : T[A]) -> Bool {
 /// assert_eq!(a, @list.from_array([1, 3, 5]))
 /// assert_eq!(b, @list.from_array([2, 4, 6]))
 /// ```
-pub fn unzip[A, B](list : T[(A, B)]) -> (T[A], T[B]) {
-  match list {
+pub fn unzip[A, B](self : T[(A, B)]) -> (T[A], T[B]) {
+  match self {
     Nil => (Nil, Nil)
     Cons((x, y), xs) => {
       let (xs, ys) = unzip(xs)

immut/list/list.mbti

@@ -10,8 +10,6 @@ fn singleton[A](A) -> T[A]
 
 fn unfold[A, S]((S) -> (A, S)?, init~ : S) -> T[A]
 
-fn unzip[A, B](T[(A, B)]) -> (T[A], T[B])
-
 // Types and methods
 pub(all) enum T {
   Nil
@@ -87,6 +85,7 @@ impl T {
   unsafe_maximum[A : Compare](Self[A]) -> A
   unsafe_minimum[A : Compare](Self[A]) -> A
   unsafe_nth[A](Self[A], Int) -> A
+  unzip[A, B](Self[(A, B)]) -> (Self[A], Self[B])
   zip[A, B](Self[A], Self[B]) -> Self[(A, B)]?
 }
 impl[A : Eq] Eq for T[A]