message: add impl of From<EpochSlotId> and From<EpochId> for BlockDate
  this will make type conversion more standard in the library
diff: @@ -86,6 +86,16 @@ impl fmt::Display for BlockDate {
  }
  }
  }
  +impl From<EpochSlotId> for BlockDate {
  + fn from(esi: EpochSlotId) -> Self {
  + BlockDate::Normal(esi)
  + }
  +}
  +impl From<EpochId> for BlockDate {
  + fn from(ei: EpochId) -> Self {
  + BlockDate::Boundary(ei)
  + }
  +}
  impl str::FromStr for BlockDate {
  type Err = BlockDateParseError;
  