message: add an EpochId type
diff: @@ -142,9 +142,11 @@ impl fmt::Display for ChainDifficulty {
  }
  }
  +pub type EpochId = u32;
  +
  #[derive(Debug, Clone)]
  pub struct SlotId {
  - pub epoch: u32,
  + pub epoch: EpochId,
  pub slotid: u32,
  }
  impl fmt::Display for SlotId {
  