Refactor list_extend() and list_fill() for gains in code size, memory

utilization, and speed:

* 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
    5 to 10% faster when x is a list or tuple
    15% faster when x is an iterable not defining __len__
    twice as fast when x is an iterable defining __len__

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

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