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.

eth2/block_proposer/src/traits.rs

@@ -1,4 +1,4 @@
-use types::{BeaconBlock, Signature, Slot};
+use types::{BeaconBlock, Fork, Signature, Slot};
 
 #[derive(Debug, PartialEq, Clone)]
 pub enum BeaconNodeError {
@@ -40,6 +40,7 @@ pub enum DutiesReaderError {
 /// Informs a validator of their duties (e.g., block production).
 pub trait DutiesReader: Send + Sync {
     fn is_block_production_slot(&self, slot: Slot) -> Result<bool, DutiesReaderError>;
+    fn fork(&self) -> Result<Fork, DutiesReaderError>;
 }
 
 /// Signs message using an internally-maintained private key.