Changed PySequence_List() and PySequence_Tuple() to support

Changed PySequence_List() and PySequence_Tuple() to support
"indefinite length" sequences.  These should still have a length, but
the length is only used as a hint -- the actual length of the sequence
is determined by the item that raises IndexError, which may be either
smaller or larger than what len() returns.  (This is a novelty; map(),
filter() and reduce() only allow the actual length to be larger than
what len() returns, not shorter.  I'll fix that shortly.)