js-on-sx: arguments object + Array.from mapFn calling convention

Three related fixes:
1. Every JS function body binds arguments to (cons p1 ... __extra_args__),
   so arguments[k] and arguments.length work as expected.
2. Array.from(iter, mapFn) invokes mapFn through js-call-with-this
   with the index as second arg (was (map-fn x), missing index and
   inheriting outer this).
3. thisArg defaults to js-global-this when omitted (per non-strict ES).
conformance.sh: 148/148.

lib/js/runtime.sx

@@ -3611,7 +3611,11 @@
         (let
           ((src (js-iterable-to-list (nth args 0)))
             (map-fn
-              (if (< (len args) 2) nil (nth args 1))))
+              (if (< (len args) 2) nil (nth args 1)))
+            (this-arg
+              (if (or (< (len args) 3) (js-undefined? (nth args 2)) (= (nth args 2) nil))
+                js-global-this
+                (nth args 2))))
           (if
             (= map-fn nil)
             (let
@@ -3623,8 +3627,9 @@
               (for-each
                 (fn
                   (x)
-                  (append! result (map-fn x))
-                  (set! i (+ i 1)))
+                  (begin
+                    (append! result (js-call-with-this this-arg map-fn (list x i)))
+                    (set! i (+ i 1))))
                 src)
               result)))))))