cut: optimize -b by avoiding per byte iteration

cut: optimize -b by avoiding per byte iteration

Always memchr(line_delim) which is fast and allows:

- skipping whole segments when the next selected byte is beyond them
- skipping unselected prefixes in bulk
- writing contiguous selected spans in bulk

This wins for lines >= 4 characters,
but is slower lines <= 3 characters, especially if selecting bytes 1-3.
That is unusual though.