message: Made fast field reader clonable.
diff: @@ -11,6 +11,7 @@ use crate::TantivyError;
  ///
  /// Internally, `FastFieldReaders` have preloaded fast field readers,
  /// and just wraps several `HashMap`.
  +#[derive(Clone)]
  pub struct FastFieldReaders {
  schema: Schema,
  fast_fields_composite: CompositeFile,
  