wrap_bytes() is similar to textwrap::wrap(), but can process arbitrary bytes.
More importantly, it guarantees that byte offsets can be reconstructed from
the split slices. This allows us to interleave push/pop_label()s with split
text fragments.
We could calculate byte offsets upfront, but using slice API is more
convenient. That's why I didn't add inner function returning Vec<Range>.
New word-wrap function will be implemented in two passes. The first pass
splits byte slice to lines, and the second pass inserts "\n" based on that
while interleaving push/pop_label() calls and text fragments. Since the second
pass combines multiple data sources, byte indices are more convenient than
slices there.
