basic List.keepIf set definition, but no implementation

compiler/builtins/src/unique.rs

@@ -783,6 +783,21 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         )
     });
 
+    // map : Attr * (List a)
+    //     , Attr Shared (a -> Attr * Bool)
+    //    -> Attr * (List b)
+    add_type(Symbol::LIST_KEEP_IF, {
+        let_tvars! { a, b, star1, star2, star3 };
+
+        unique_function(
+            vec![
+                list_type(star1, a),
+                shared(SolvedType::Func(vec![flex(a)], Box::new(bool_type(star2)))),
+            ],
+            list_type(star3, b),
+        )
+    });
+
     // foldr : Attr (* | u) (List (Attr u a))
     //       , Attr Shared (Attr u a -> b -> b)
     //       , b