Adds fmt::Debug for dyn SerialPort and related enums.

[nathansizemore]

Helpful in circumstances where you want to extract the error from an open and dip out:

let r = serialport::new().open();
if r.is_err() {
    let e = r.unwrap_err(); // dyn SerialPort does not implement fmt::Debug
}

[eldruin]

Sounds like a good idea, thank you!
However, the current version does not build in the MSRV version because the format string capture used is only available in Rust >= 1.58.0. Could you change that to make it compatible?

[eldruin]

Thank you!