message: Fix Clippy lint.
  Clippy complains about `Block::take_header` being unused, but only if
  compiled without fast sync. Instead of conditionally compiling it, I
  made the method `pub`, since it does make sense as a method on `Block`
  in general and is not specific to syncing.
diff: @@ -1065,7 +1065,8 @@ impl Block {
  &self.body
  }
  - pub(crate) fn take_header(self) -> BlockHeader {
  + /// Returns the header, consuming the block.
  + pub fn take_header(self) -> BlockHeader {
  self.header
  }
  