* Moved the responsibility for emptying the previous list from list_fill

* Moved the responsibility for emptying the previous list from list_fill
  to list_init.

* Replaced the code in list_extend with the superior code from list_fill.

* Eliminated list_fill.

Results:

* list.extend() no longer creates an intermediate tuple except to handle
  the special case of x.extend(x).  The saves memory and time.

* list.extend(x) runs
    about the same x is a list or tuple,
    a little faster when x is an iterable not defining __len__, and
    twice as fast when x is an iterable defining __len__.

* the code is about 15 lines shorter and no longer duplicates
  functionality.