message: expose MessageId
diff: @@ -21,7 +21,7 @@ pub use self::header::{
  GenesisPraosProof, Header, HeaderHash, KESSignature, Proof,
  };
  pub use self::headerraw::HeaderRaw;
  -pub use self::message::{Message, MessageRaw};
  +pub use self::message::{Message, MessageRaw, MessageId};
  pub use self::version::*;
  pub use crate::date::{BlockDate, BlockDateParseError};
  