`Writer::write_str` does an infinite loop if a line is longer than line_len

```rust while self.current_line + line.len() >= self.cfg.line_len { let offset = if let Some(last_ws) = line[..self.cfg.line_len - self.current_line] .chars() .rev() .position(|c| c.is_whitespace()) { // found a nice whitespace to break on! self.writer.write_str(&line[..last_ws])?; last_ws } else { 0 }; self.writer.write_char('\n')?; self.write_newline()?; self.writer.write_str(" ")?; self.current_line += 1; line = &line[offset..]; } ``` if there's no whitespace in `line[..self.cfg.line_len - self.current_line]` we uh, advance by zero, write a newline, then try again. i held down `z` for a while to see if the new 16550 code does anything weird on faster inputs and noticed uh, this. on one hand, https://github.com/hawkw/mycelium/issues/493, on the other hand, the wrapping probably makes sense for non-serial-port readers (like, it's better to wrap smartly in the framebuffer than dumbly?)