message: Add a method to construct a block from its header.
diff: @@ -834,6 +834,18 @@ impl Block {
  }
  }
  + /// Creates an instance of the block from the block header.
  + pub(crate) fn from_header(header: BlockHeader) -> Self {
  + let body = ();
  + let hash = header.hash();
  + Block {
  + hash,
  + header,
  + body,
  + proofs: BTreeMap::new(),
  + }
  + }
  +
  pub(crate) fn header(&self) -> &BlockHeader {
  &self.header
  }
  