tuples: change to abstract iteration.

* lib.c (make_like): In the COBJ case, recognize an iterator
object. Pull out the underlying object and recurse on it.
This is needed in tuples_func, where make_like will now be
called on the abstract iterator, rather than the actual
sequence object.
(tuples_func): The incoming object is now an iterator, and not
a sequence; we need to handle it with iter_more, iter_item and
iter_step.
(tuples): Instead of nullify, begin iteration with
iter_begin, and use iter_more to test for empty.
In non-empty case, put propagate the iterator thorugh the lazy
cons car field, rather than the sequence.

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