js-on-sx: constructors carry __proto__ = Function.prototype

Object/Array/Number/String/Boolean had no __proto__, so
Function.prototype mutations were invisible to them. Added a
post-init (begin (dict-set! ...)) at the end of runtime.sx
that wires each constructor to js-function-global.prototype.
Combined with the recent Object.prototype fallback, the chain
now terminates correctly: ctor → Function.prototype → Object.prototype.
built-ins/Number: 41/50 → 42/50, built-ins/String: 75/99 → 78/99,
built-ins/Array: 12/45 → 13/45. conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,26 +1,26 @@
 {
   "totals": {
-    "pass": 77,
-    "fail": 16,
+    "pass": 78,
+    "fail": 15,
     "skip": 1,
     "timeout": 6,
     "total": 100,
     "runnable": 99,
-    "pass_rate": 77.8
+    "pass_rate": 78.8
   },
   "categories": [
     {
       "category": "built-ins/String",
       "total": 100,
-      "pass": 77,
-      "fail": 16,
+      "pass": 78,
+      "fail": 15,
       "skip": 1,
       "timeout": 6,
-      "pass_rate": 77.8,
+      "pass_rate": 78.8,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          14
+          13
         ],
         [
           "Timeout",
@@ -40,7 +40,7 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      14
+      13
     ],
     [
       "Timeout",
@@ -56,6 +56,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 250.0,
+  "elapsed_seconds": 273.6,
   "workers": 1
 }