Add transparent attribute for delegating Error impl to one field

[dtolnay]

This is useful for hiding error variants from a library's public error type:

#[derive(Error, Debug)]
#[error(transparent)]  // source and Display delegate to ErrorKind
pub struct Error(ErrorKind);

#[derive(Error, Debug)]
/*private*/ enum ErrorKind {
    #[error("...")]
    E0,
    #[error("...")]
    E1(#[source] io::Error),
}

And also for enums that need an "anything else" variant; such variants tend not to have their own Display message but just forward through to the underlying error's Display and source:

#[derive(Error, Debug)]
pub enum MyError {
    ...

    #[error(transparent)]
    Other(#[from] anyhow::Error),  // source and Display delegate to anyhow::Error
}

Closes #40.