Forward-port of r52136,52138: a review of overflow-detecting code.

* unified the way intobject, longobject and mystrtoul handle
  values around -sys.maxint-1.

* in general, trying to entierely avoid overflows in any computation
  involving signed ints or longs is extremely involved.  Fixed a few
  simple cases where a compiler might be too clever (but that's all
  guesswork).

* more overflow checks against bad data in marshal.c.

* 2.5 specific: fixed a number of places that were still confusing int
  and Py_ssize_t.  Some of them could potentially have caused
  "real-world" breakage.

* list.pop(x): fixing overflow issues on x was messy.  I just reverted
  to PyArg_ParseTuple("n"), which does the right thing.  (An obscure
  test was trying to give a Decimal to list.pop()... doesn't make
  sense any more IMHO)

* trying to write a few tests...

Lib/test/test_long.py

@@ -247,17 +247,23 @@ class LongTest(unittest.TestCase):
             "long(-sys.maxint-1) != -sys.maxint-1")
 
         # long -> int should not fail for hugepos_aslong or hugeneg_aslong
+        x = int(hugepos_aslong)
         try:
-            self.assertEqual(int(hugepos_aslong), hugepos,
+            self.assertEqual(x, hugepos,
                   "converting sys.maxint to long and back to int fails")
         except OverflowError:
             self.fail("int(long(sys.maxint)) overflowed!")
+        if not isinstance(x, int):
+            raise TestFailed("int(long(sys.maxint)) should have returned int")
+        x = int(hugeneg_aslong)
         try:
-            self.assertEqual(int(hugeneg_aslong), hugeneg,
+            self.assertEqual(x, hugeneg,
                   "converting -sys.maxint-1 to long and back to int fails")
         except OverflowError:
             self.fail("int(long(-sys.maxint-1)) overflowed!")
-
+        if not isinstance(x, int):
+            raise TestFailed("int(long(-sys.maxint-1)) should have "
+                             "returned int")
         # but long -> int should overflow for hugepos+1 and hugeneg-1
         x = hugepos_aslong + 1
         try:
@@ -282,6 +288,17 @@ class LongTest(unittest.TestCase):
         self.assert_(type(y) is long,
             "overflowing int conversion must return long not long subtype")
 
+        # long -> Py_ssize_t conversion
+        class X(object):
+            def __getslice__(self, i, j):
+                return i, j
+
+        self.assertEqual(X()[-5L:7L], (-5, 7))
+        # use the clamping effect to test the smallest and largest longs
+        # that fit a Py_ssize_t
+        slicemin, slicemax = X()[-2L**100:2L**100]
+        self.assertEqual(X()[slicemin:slicemax], (slicemin, slicemax))
+
 # ----------------------------------- tests of auto int->long conversion
 
     def test_auto_overflow(self):