Add Eventuality back to processor primitives

Also splits crate into modules.

processor/primitives/src/eventuality.rs

@@ -0,0 +1,31 @@
+use std::collections::HashMap;
+use std::io;
+
+/// A description of a transaction which will eventually happen.
+pub trait Eventuality: Sized + Send + Sync {
+  /// A unique byte sequence which can be used to identify potentially resolving transactions.
+  ///
+  /// Both a transaction and an Eventuality are expected to be able to yield lookup sequences.
+  /// Lookup sequences MUST be unique to the Eventuality and identical to any transaction's which
+  /// satisfies this Eventuality. Transactions which don't satisfy this Eventuality MAY also have
+  /// an identical lookup sequence.
+  ///
+  /// This is used to find the Eventuality a transaction MAY resolve so we don't have to check all
+  /// transactions against all Eventualities. Once the potential resolved Eventuality is
+  /// identified, the full check is performed.
+  fn lookup(&self) -> Vec<u8>;
+
+  /// Read an Eventuality.
+  fn read<R: io::Read>(reader: &mut R) -> io::Result<Self>;
+  /// Serialize an Eventuality to a `Vec<u8>`.
+  fn serialize(&self) -> Vec<u8>;
+}
+
+/// A tracker of unresolved Eventualities.
+#[derive(Debug)]
+pub struct EventualityTracker<E: Eventuality> {
+  /// The active Eventualities.
+  ///
+  /// These are keyed by their lookups.
+  pub active_eventualities: HashMap<Vec<u8>, E>,
+}