js-on-sx: String(arr) honours Array.prototype.toString overrides

Was always emitting comma-joined via js-list-join, so user
mutations of Array.prototype.toString had no effect on String(arr)
/ "" + arr. Now look up the override via js-dict-get-walk and call
it on the list as this; fall back to (js-list-join v ",") when the
override doesn't return a string.
String fail count: 11 → 9. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -1402,7 +1402,15 @@
                       (js-to-string result)))
                   "[object Object]"))))
           (cond
-            ((= (type-of v) "list") (js-list-join v ","))
+            ((= (type-of v) "list")
+              (let
+                ((tostr-fn (js-dict-get-walk (get Array "prototype") "toString")))
+                (if
+                  (= (type-of tostr-fn) "lambda")
+                  (let
+                    ((result (js-call-with-this v tostr-fn ())))
+                    (if (= (type-of result) "string") result (js-list-join v ",")))
+                  (js-list-join v ","))))
             ((js-function? v)
               (let
                 ((tostr-fn (js-dict-get-walk (get js-function-global "prototype") "toString")))