message: Add a public way to create a FileSpan
  Summary: Useful in a later diff. Seems a reasonable way to complete the API.
diff: @@ -450,6 +450,13 @@ impl<'a> FileSpanRef<'a> {
  }
  impl FileSpan {
  + /// Creates an new `FileSpan` covering the entire file.
  + pub fn new(filename: String, source: String) -> Self {
  + let file = CodeMap::new(filename, source);
  + let span = file.full_span();
  + Self { file, span }
  + }
  +
  /// Filename of this span.
  pub fn filename(&self) -> &str {
  self.file.filename()
  