js-on-sx: Object.prototype.toString dispatches by [[Class]]

Was hardcoded to "[object Object]" for everything; per ES it should
return "[object Array]", "[object Function]", "[object Number]",
etc. by class. Added js-object-tostring-class helper that switches
on type-of and dict-internal markers (__js_*_value__,
__callable__). Prototype-identity checks ensure
Object.prototype.toString.call(Number.prototype) returns
"[object Number]" (similar for String/Boolean/Array).
built-ins/Array: 18/45 → 20/45, built-ins/Number: 43/50 → 44/50.
conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,27 +1,45 @@
 {
   "totals": {
-    "pass": 45,
-    "fail": 0,
-    "skip": 5,
-    "timeout": 0,
+    "pass": 44,
+    "fail": 3,
+    "skip": 0,
+    "timeout": 3,
     "total": 50,
-    "runnable": 45,
-    "pass_rate": 100.0
+    "runnable": 50,
+    "pass_rate": 88.0
   },
   "categories": [
     {
-      "category": "built-ins/Math",
+      "category": "built-ins/Number",
       "total": 50,
-      "pass": 45,
-      "fail": 0,
-      "skip": 5,
-      "timeout": 0,
-      "pass_rate": 100.0,
-      "top_failures": []
+      "pass": 44,
+      "fail": 3,
+      "skip": 0,
+      "timeout": 3,
+      "pass_rate": 88.0,
+      "top_failures": [
+        [
+          "Timeout",
+          3
+        ],
+        [
+          "Test262Error (assertion failed)",
+          3
+        ]
+      ]
     }
   ],
-  "top_failure_modes": [],
+  "top_failure_modes": [
+    [
+      "Timeout",
+      3
+    ],
+    [
+      "Test262Error (assertion failed)",
+      3
+    ]
+  ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 9.0,
+  "elapsed_seconds": 57.4,
   "workers": 1
 }