Bug 904723, part 1 - Add new IsConstructor intrinsic for self-hosting. r=till.

This also renames js_IsCallable to js::IsCallable and does some related light
cleaning work.

js/src/jsdate.cpp

@@ -2479,7 +2479,7 @@ date_toJSON(JSContext *cx, unsigned argc, Value *vp)
         return false;
 
     /* Step 5. */
-    if (!js_IsCallable(toISO)) {
+    if (!IsCallable(toISO)) {
         JS_ReportErrorFlagsAndNumber(cx, JSREPORT_ERROR, js_GetErrorMessage, nullptr,
                                      JSMSG_BAD_TOISOSTRING_PROP);
         return false;