Implement #[error(fmt = ...)]

[dtolnay]

Before:

#[derive(Error, Debug)]
pub enum Error {
    #[error("{code}{}", match .message {
        Some(msg) => format!(" - {}", &msg),
        None => "".to_owned(),
    })]
    Demo { code: u16, message: Option<String> },
}

After:

#[derive(Error, Debug)]
pub enum Error {
    #[error(fmt = demo_fmt)]
    Demo { code: u16, message: Option<String> },
}

fn demo_fmt(code: &u16, message: &Option<String>, formatter: &mut fmt::Formatter) -> fmt::Result {
    write!(formatter, "{code}")?;
    if let Some(msg) = message {
        write!(formatter, " - {msg}")?;
    }
    Ok(())
}

This avoids forcing allocations (or the even worse workarounds in #201) and keeps elaborate formatting logic out of the data structure.

The parameter order is designed to be usable with generic methods.

#[derive(Error, Debug)]
#[error(fmt = core::fmt::Octal::fmt)]
pub enum Error {
    I32(i32),
    I64(i64),
}