Mondo changes to the iterator stuff, without changing how Python code

sees it (test_iter.py is unchanged).

- Added a tp_iternext slot, which calls the iterator's next() method;
  this is much faster for built-in iterators over built-in types
  such as lists and dicts, speeding up pybench's ForLoop with about
  25% compared to Python 2.1.  (Now there's a good argument for
  iterators. ;-)

- Renamed the built-in sequence iterator SeqIter, affecting the C API
  functions for it.  (This frees up the PyIter prefix for generic
  iterator operations.)

- Added PyIter_Check(obj), which checks that obj's type has a
  tp_iternext slot and that the proper feature flag is set.

- Added PyIter_Next(obj) which calls the tp_iternext slot.  It has a
  somewhat complex return condition due to the need for speed: when it
  returns NULL, it may not have set an exception condition, meaning
  the iterator is exhausted; when the exception StopIteration is set
  (or a derived exception class), it means the same thing; any other
  exception means some other error occurred.

Include/object.h

@@ -201,6 +201,7 @@ typedef PyObject *(*reprfunc)(PyObject *);
 typedef long (*hashfunc)(PyObject *);
 typedef PyObject *(*richcmpfunc) (PyObject *, PyObject *, int);
 typedef PyObject *(*getiterfunc) (PyObject *);
+typedef PyObject *(*iternextfunc) (PyObject *);
 
 typedef struct _typeobject {
 	PyObject_VAR_HEAD
@@ -252,6 +253,7 @@ typedef struct _typeobject {
 
 	/* Iterators */
 	getiterfunc tp_iter;
+	iternextfunc tp_iternext;
 
 #ifdef COUNT_ALLOCS
 	/* these must be last and never explicitly initialized */