This might have output that is a little better in cases where the
wrapped string is longer than `maxlen', for example, with an input of:
" x xxxxxxxxxxxxxxxx..."
and wrapping at 10 characters, the output was
" x\nxxxxxxxxxx\nxxxxxx..."
and now it's
" x xxxx\nxxxxxxxxxx\nxx..."
This should probably become documented if it's stable enough. For now
it's intended for building wrapping functions, so it's only documented
in comments the file.