Add TreeHash derives for cache objects.

This allows us to avoid a verbose manual impl for BeaconState

eth2/types/src/beacon_state/epoch_cache.rs

@@ -1,5 +1,7 @@
 use super::{AttestationDuty, BeaconState, CrosslinkCommittees, Error};
+use crate::test_utils::TestRandom;
 use crate::{ChainSpec, Epoch};
+use rand::RngCore;
 use serde_derive::{Deserialize, Serialize};
 
 #[derive(Debug, Default, PartialEq, Clone, Serialize, Deserialize)]
@@ -67,3 +69,14 @@ impl EpochCache {
         })
     }
 }
+
+impl<T: RngCore> TestRandom<T> for [EpochCache; 3] {
+    /// Test random should generate an empty cache.
+    fn random_for_test(rng: &mut T) -> Self {
+        [
+            EpochCache::default(),
+            EpochCache::default(),
+            EpochCache::default(),
+        ]
+    }
+}

eth2/types/src/beacon_state/pubkey_cache.rs

@@ -1,4 +1,6 @@
+use crate::test_utils::TestRandom;
 use crate::*;
+use rand::RngCore;
 use serde_derive::{Deserialize, Serialize};
 use std::collections::HashMap;
 
@@ -36,3 +38,10 @@ impl PubkeyCache {
         self.map.get(pubkey).cloned()
     }
 }
+
+impl<T: RngCore> TestRandom<T> for PubkeyCache {
+    /// Test random should generate an empty cache.
+    fn random_for_test(rng: &mut T) -> Self {
+        Self::default()
+    }
+}