message: mockchain: Add Display impl for BlockDate
diff: //! Representation of the block in the mockchain.
  -use crate::key::*;
  +use crate::key::{Hash, PrivateKey, PublicKey, Signature};
  use crate::transaction::*;
  use chain_core::property;
  +use std::fmt;
  +
  /// Non unique identifier of the transaction position in the
  /// blockchain. There may be many transactions related to the same
  /// `SlotId`.
  @@ -99,6 +101,12 @@ impl property::BlockDate for BlockDate {
  }
  }
  +impl fmt::Display for BlockDate {
  + fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  + write!(f, "{}.{}", self.epoch, self.slot_id)
  + }
  +}
  +
  impl property::Block for Block {
  type Id = Hash;
  type Date = BlockDate;
  