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).

Avoid checks for negative indices and duplicate checks for support of
the sequence protocol.

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.