UTF-8 -emitting fmt::Write wrapper for io::Write

fmt_write.rs

struct FmtWriter<W: std::io::Write>(W);

impl <W: std::io::Write> std::fmt::Write for FmtWriter<W> {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        self.0.write_all(s.as_bytes()).map_err(|_| fmt::Error)
    }
}