[consensus] Emit Nullify and Nullification Proofs

[patrick-ogrady]

Some applications make use of nullify and nullification messages. We should expose them (like we do the other activity):

• add NULLIFY:

  monorepo/consensus/src/simplex/mod.rs

  Lines 162 to 177 in cee6569

  	/// Notarize a payload at a given view.
  	///
  	/// ## Clarifications
  	/// * Vote for leader is considered a proposal and a vote.
  	/// * It is ok to have both a vote for a proposal and the null
  	/// container in the same view.
  	/// * It is ok to notarize/finalize different proposals in the same view.
  	pub const NOTARIZE: Activity = 0;
  	/// Finalize a payload at a given view.
  	pub const FINALIZE: Activity = 1;
  	/// Notarize a payload that conflicts with a previous notarize.
  	pub const CONFLICTING_NOTARIZE: Activity = 2;
  	/// Finalize a payload that conflicts with a previous finalize.
  	pub const CONFLICTING_FINALIZE: Activity = 3;
  	/// Nullify and finalize in the same view.
  	pub const NULLIFY_AND_FINALIZE: Activity = 4;

• add skipped:

  monorepo/consensus/src/lib.rs

  Lines 89 to 100 in cee6569

  	/// Committer is the interface responsible for handling notifications of payload status.
  	pub trait Committer: Clone + Send + 'static {
  	/// Digest is an arbitrary hash digest.
  	type Digest: Digest;
  	/// Event that a payload has made some progress towards finalization but is not yet finalized.
  	///
  	/// This is often used to provide an early ("best guess") confirmation to users.
  	fn prepared(&mut self, proof: Proof, payload: Self::Digest) -> impl Future<Output = ()> + Send;
  	/// Event indicating the container has been finalized.
  	fn finalized(&mut self, proof: Proof, payload: Self::Digest) -> impl Future<Output = ()> + Send;

[najeal]

@patrick-ogrady 👋