message: [pdsl_core] Add MutChunk::replace
diff: @@ -386,6 +386,21 @@ impl<T> MutChunk<T>
  where
  T: parity_codec::Codec
  {
  + /// Sets the value of the `n`-th cell and returns its old value if any.
  + ///
  + /// # Note
  + ///
  + /// Use [`set`](struct.MutChunk.html#method.set) instead
  + /// if you are not interested in the old return value.
  + ///
  + /// # Errors
  + ///
  + /// If `n` is out of bounds.
  + #[must_use]
  + pub fn replace(&mut self, n: u32, val: T) -> Result<Option<T>> {
  + self.cell_at(n)
  + .map(|cell| cell.replace(val))
  + }
  /// Mutates the value of the `n`-th cell if any.
  ///
  