js-on-sx: for..of / for..in + more Array methods

Parser: jp-parse-for-stmt does 2-token lookahead for (var? ident
(of|in) expr), emits (js-for-of-in kind ident iter body) else
classic (js-for init cond step body).

Transpile: wraps body in (call/cc (__break__) (let items
(for-each (fn (ident) (call/cc (__continue__) body)) items))).

Runtime: js-iterable-to-list normalizes list/string/dict for of
iteration; js-string-to-list expands string to char list.

399/401 unit (+8), 148/148 slice unchanged.

lib/js/runtime.sx

@@ -1585,6 +1585,29 @@
           (dict-set! p "value" reason)
           (js-promise-flush-callbacks! p))))))
 
+(define
+  js-iterable-to-list
+  (fn
+    (v)
+    (cond
+      ((list? v) v)
+      ((= (type-of v) "string") (js-string-to-list v 0 (list)))
+      ((dict? v)
+        (let
+          ((result (list)))
+          (for-each (fn (k) (append! result (get v k))) (keys v))
+          result))
+      (else (list)))))
+
+(define
+  js-string-to-list
+  (fn
+    (s i acc)
+    (if
+      (>= i (len s))
+      acc
+      (begin (append! acc (char-at s i)) (js-string-to-list s (+ i 1) acc)))))
+
 (define
   js-object-keys
   (fn