Lists: add missing elemAt and fix map's type
length : List -> Num = fun l => %length% l;
- map : forall a b. (a -> b) -> List -> List = fun f l => %map% f l;
+ map : (Dyn -> Dyn) -> List -> List = fun f l => %map% f l;
+
+ elemAt : List -> Num -> Dyn = fun l n => %elemAt% l n;
concat : List -> List -> List = fun l1 l2 => l1 @ l2;
