Convert iterator __len__() methods to a private API.

Lib/test/test_iterlen.py

@@ -43,12 +43,22 @@ enumerate(iter('abc')).
 
 import unittest
 from test import test_support
-from itertools import repeat, count
+from itertools import repeat
 from collections import deque
 from UserList import UserList
+from __builtin__ import len as _len
 
 n = 10
 
+def len(obj):
+    try:
+        return _len(obj)
+    except TypeError:
+        try:
+            return obj._length_cue()
+        except AttributeError:
+            raise TypeError
+
 class TestInvariantWithoutMutations(unittest.TestCase):
 
     def test_invariant(self):