Fix buglet reported on c.l.py: map(fnc, file.xreadlines()) blows up.

Fix buglet reported on c.l.py:  map(fnc, file.xreadlines()) blows up.
Also a 2.1 bugfix candidate (am I supposed to do something with those?).
Took away map()'s insistence that sequences support __len__, and cleaned
up the convoluted code that made it *look* like it really cared about
__len__ (in fact the old ->len field was only *used* as a flag bit, as
the main loop only looked at its sign bit, setting the field to -1 when
IndexError got raised; renamed the field to ->saw_IndexError instead).