Support mixing shorthand and non-shorthand format args

[dtolnay]

As in:

mod msg {
    pub const ERR_INVALID_UTF8: &str = "argument not convertible to UTF-8";
}

#[derive(Error, Debug)]
pub enum Error {
    #[error("{}: {value:?}", msg::ERR_INVALID_UTF8)]
    InvalidUtf8 { value: std::ffi::OsString },
}

Currently this fails because, as documented in the readme, our "{value:?}" shorthand does not kick in if any explicit format args are specified.

[dtolnay]

The current heuristic is:

• If the user provided any explicit format args, make no change.
• Otherwise for each {var} in the format string change it to {} and append , var to the args.

The heuristic instead would be:

• For each {var} in the format string append , var = var to the args.