message: document why it is _acceptable_ to have an unwrap here.
  relates to
diff: @@ -151,10 +151,16 @@ impl BlockHeader {
  }
  pub fn to_raw(&self) -> RawBlockHeader {
  + // the only reason this would fail is if there was no more memory
  + // to allocate. This would be the users' last concern if it was the
  + // case
  RawBlockHeader(cbor!(self).unwrap())
  }
  pub fn compute_hash(&self) -> HeaderHash {
  + // the only reason this would fail is if there was no more memory
  + // to allocate. This would be the users' last concern if it was the
  + // case
  let v = cbor!(self).unwrap();
  HeaderHash::new(&v[..])
  }
  