message: add dummy apply function
diff: //! The global ledger/update/delegation states
  //!
  +use crate::block::BlockContents;
  use crate::{account, leadership, setting, utxo};
  use cardano::address::Addr as OldAddress;
  use chain_addr::Address;
  @@ -26,3 +27,9 @@ pub struct State {
  pub(crate) settings: setting::Settings,
  pub(crate) leadership: Leadership,
  }
  +
  +impl State {
  + pub fn apply(&self, content: BlockContents) -> State {
  + unimplemented!()
  + }
  +}
  