message: add SlotId next and derive some traits
diff: @@ -124,11 +124,16 @@ impl fmt::Display for ChainDifficulty {
  pub type EpochId = u32;
  -#[derive(Debug, Clone)]
  +#[derive(Debug, Clone, PartialEq, Eq)]
  pub struct SlotId {
  pub epoch: EpochId,
  pub slotid: u32,
  }
  +impl SlotId {
  + pub fn next(&self) -> Self {
  + SlotId { epoch: self.epoch, slotid: self.slotid + 1 }
  + }
  +}
  impl fmt::Display for SlotId {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  write!(f, "{}.{}", self.epoch, self.slotid)
  