message: simplify Coordinator::update_dict_mask
diff: use std::{
  collections::{HashMap, HashSet},
  default::Default,
  + iter,
  };
  use counter::Counter;
  @@ -242,9 +243,7 @@ impl Coordinator {
  /// Update the mask dictionary.
  fn update_dict_mask(&mut self, mask_hash: &[u8]) {
  - self.dict_mask += mask_hash
  - .chunks_exact(mask_hash.len())
  - .map(|mask| mask.to_vec());
  + self.dict_mask.update(iter::once(mask_hash.to_vec()))
  }
  /// Freeze the mask dictionary. Returns a unique mask. Fails due to insufficient sum
  