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/transpile.sx

@@ -940,6 +940,21 @@
           (js-param-sym (first params))
           (js-build-param-list (rest params)))))))
 
+(define
+  js-arguments-build-form
+  (fn
+    (params)
+    (cond
+      ((empty? params)
+        (js-sym "__extra_args__"))
+      ((and (list? (first params)) (js-tag? (first params) "js-rest"))
+        (js-sym (nth (first params) 1)))
+      (else
+        (list
+          (js-sym "cons")
+          (js-param-sym (first params))
+          (js-arguments-build-form (rest params)))))))
+
 (define
   js-param-init-forms
   (fn
@@ -1402,7 +1417,9 @@
         param-syms
         (list
           (js-sym "let")
-          (list (list (js-sym "this") (list (js-sym "js-this"))))
+          (list
+            (list (js-sym "this") (list (js-sym "js-this")))
+            (list (js-sym "arguments") (js-arguments-build-form params)))
           (list
             (js-sym "let")
             (list