(Contributed by Dima Dorfman)

* Document the differences between them
* Fix corner cases covered by the unittests
* Use Py_RETURN_NONE where possible for dictionaries

to more accurately describe what the function does.

Suggested by Thomas Wouters.

Factors out the common case of returning self.

-- replace then with slightly faster PyObject_Call(o,a,NULL).  (The
difference is that the latter requires a to be a tuple; the former
allows other values and wraps them in a tuple if necessary; it
involves two more levels of C function calls to accomplish all that.)

tupleobject.c. Makes the code in iterobject.c cleaner
and speeds-up the general case by not checking for
tuples everytime.   SF Patch #592065.

object references (it_seq for seqiterobject, it_callable and
it_sentinel for calliterobject) when the end of the list is reached.

Also remove the next() methods -- one is supplied automatically by
PyType_Ready() because the tp_iternext slot is set.  That's a good
thing, because the implementation given here was buggy (it never
raised StopIteration).