Performance regression due to width_aware_slice #717

Apparently ef91741 introduced a significant performance regression when printing a long output. (The precise commit was pinned down through git bisect.)

To reproduce it, run

$ bpython
bpython version 0.18.dev15 on top of Python 2.7.13 /tmp/bpython/.venv/bin/python2
>>> ' '.join(map(str, xrange(10000)))
(hangs)

After reverting ef91741, printing ' '.join(map(str, xrange(10000))) takes less than a blink.