js-on-sx: built-in .length returns spec-defined values

String.fromCharCode.length, Math.max.length, Array.from.length
were returning 0 because their SX lambdas use &rest args with no
required params — but spec assigns each a specific length.
Added js-builtin-fn-length mapping JS name to spec length (12
entries). js-fn-length consults the table first and falls back to
counting real params.
built-ins/String: 79/99 → 80/99, built-ins/Array: 20/45 → 21/45.
conformance.sh: 148/148.

lib/js/runtime.sx

@@ -113,13 +113,35 @@
     (let
       ((t (type-of f)))
       (cond
-        ((= t "lambda") (js-count-real-params (lambda-params f)))
+        ((= t "lambda")
+          (let
+            ((mapped (js-builtin-fn-length (js-unmap-fn-name (js-extract-fn-name f)))))
+            (if (>= mapped 0) mapped (js-count-real-params (lambda-params f)))))
         ((= t "function") 0)
         ((= t "component") 0)
         ((and (= t "dict") (contains? (keys f) "__callable__"))
           (js-fn-length (get f "__callable__")))
         (else 0)))))
 
+(define
+  js-builtin-fn-length
+  (fn
+    (name)
+    (cond
+      ((= name "fromCharCode") 1)
+      ((= name "fromCodePoint") 1)
+      ((= name "raw") 1)
+      ((= name "of") 0)
+      ((= name "from") 1)
+      ((= name "isArray") 1)
+      ((= name "max") 2)
+      ((= name "min") 2)
+      ((= name "hypot") 2)
+      ((= name "atan2") 2)
+      ((= name "imul") 2)
+      ((= name "pow") 2)
+      (else -1))))
+
 (define
   js-extract-fn-name
   (fn