Optimize "{0}" and "{var}" to not require write!

[dtolnay]

#12 called out that the generated code from "{0}" is messier than just a Display::fmt(_0, formatter). I don't know why write! doesn't handle this case better, but we might as well do it in thiserror.

#[derive(Error, Debug)]
pub enum ParseFromEnvError {
    #[error("{0}")]
    InvalidLogFormat(io::Error),
}

^ Should expand the Display impl to:

  match self {
-     ParseFromEnvError::InvalidLogFormat(_0) => write!(formatter, "{}", _0),
+     ParseFromEnvError::InvalidLogFormat(_0) => Display::fmt(_0, formatter),
  }

@MOZGIII

[MOZGIII]

This definetly does the right thing. I still don't like #[error("{0}")] though. How about adding #[error_proxy] or something of that sort?