Rewrite cut_characters

This follows the cut_bytes() approach of letting read_line() create a
buffer and find the newline. read_line() guarantees our buffer is a
string of utf8 characters.

When writing out the bytes segment we need to make sure we are cutting
on utf8 boundaries, there for we must iterate over the buffer
from read_line(). This implementation is(/should be) efficient as it
only iterates once over the buffer.

The previous performance was about 4x as slow as cut_bytes() and now it
is about 2x as slow as cut_bytes().