Decide flow between scan/eventuality/report

Scan now only handles External outputs, with an associated essay going over
why. Scan directly creates the InInstruction (prior planned to be done in
Report), and Eventuality is declared to end up yielding the outputs.

That will require making the Eventuality flow two-stage. One stage to evaluate
existing Eventualities and yield outputs, and one stage to incorporate new
Eventualities before advancing the scan window.

processor/scanner/src/eventuality.rs

@@ -109,6 +109,8 @@ impl<D: Db, S: ScannerFeed> ContinuallyRan for EventualityTask<D, S> {
 
       iterated = true;
 
+      // TODO: Not only check/clear eventualities, if this eventuality forwarded an output, queue
+      // it to be reported in however many blocks
       todo!("TODO");
 
       let mut txn = self.db.txn();