message: Make CodeMap derive Dupe and Clone
  Summary: It's an Arc, so make it cheap to clone.
diff: @@ -132,7 +132,7 @@ impl<T> Deref for Spanned<T> {
  }
  /// A data structure recording a source code file for position lookup.
  -#[derive(Debug)]
  +#[derive(Debug, Clone, Dupe)]
  pub struct CodeMap {
  file: Arc<File>,
  }
  