Fixes bug in epoch processing.

- Was using the wrong slot to determine relative epoch.
- Added a non-related test I build during the search

eth2/types/src/slot_epoch.rs

@@ -113,6 +113,16 @@ mod epoch_tests {
 
     all_tests!(Epoch);
 
+    #[test]
+    fn epoch_start_end() {
+        let slots_per_epoch = 8;
+
+        let epoch = Epoch::new(0);
+
+        assert_eq!(epoch.start_slot(slots_per_epoch), Slot::new(0));
+        assert_eq!(epoch.end_slot(slots_per_epoch), Slot::new(7));
+    }
+
     #[test]
     fn slot_iter() {
         let slots_per_epoch = 8;