Introduce Fork struct to block_producer

It's a pretty crappy solution, IMO. It shouldn't really belong in
"duties" but this gets the job done for now.

validator_client/src/block_producer_service/mod.rs

@@ -50,6 +50,9 @@ impl<T: SlotClock, U: BeaconNode, V: DutiesReader, W: Signer> BlockProducerServi
                 Ok(BlockProducerPollOutcome::ValidatorIsUnknown(slot)) => {
                     error!(self.log, "The Beacon Node does not recognise the validator"; "slot" => slot)
                 }
+                Ok(BlockProducerPollOutcome::UnableToGetFork(slot)) => {
+                    error!(self.log, "Unable to get a `Fork` struct to generate signature domains"; "slot" => slot)
+                }
             };
 
             std::thread::sleep(Duration::from_millis(self.poll_interval_millis));

validator_client/src/duties/epoch_duties.rs

@@ -1,7 +1,7 @@
 use block_proposer::{DutiesReader, DutiesReaderError};
 use std::collections::HashMap;
 use std::sync::RwLock;
-use types::{Epoch, Slot};
+use types::{Epoch, Fork, Slot};
 
 /// The information required for a validator to propose and attest during some epoch.
 ///
@@ -75,6 +75,17 @@ impl DutiesReader for EpochDutiesMap {
             .ok_or_else(|| DutiesReaderError::UnknownEpoch)?;
         Ok(duties.is_block_production_slot(slot))
     }
+
+    fn fork(&self) -> Result<Fork, DutiesReaderError> {
+        // TODO: this is garbage data.
+        //
+        // It will almost certainly cause signatures to fail verification.
+        Ok(Fork {
+            previous_version: 0,
+            current_version: 0,
+            epoch: Epoch::new(0),
+        })
+    }
 }
 
 // TODO: add tests.