message: add a way to compute hash from block header
diff: @@ -26,6 +26,11 @@ impl BlockHeader {
  &BlockHeader::MainBlockHeader(ref blo) => blo.consensus.slot_id.clone(),
  }
  }
  +
  + pub fn compute_hash(&self) -> HeaderHash {
  + let v = cbor::encode_to_cbor(self).unwrap();
  + HeaderHash::new(&v[..])
  + }
  }
  impl fmt::Display for BlockHeader {
  