Implement tree hash caching (#584)

* Implement basic tree hash caching

* Use spaces to indent top-level Cargo.toml

* Optimize BLS tree hash by hashing bytes directly

* Implement tree hash caching for validator registry

* Persist BeaconState tree hash cache to disk

* Address Paul's review comments

eth2/utils/bls/src/public_key_bytes.rs

@@ -6,8 +6,7 @@ bytes_struct!(
     PublicKeyBytes,
     PublicKey,
     BLS_PUBLIC_KEY_BYTE_SIZE,
-    "public key",
-    U48
+    "public key"
 );
 
 #[cfg(test)]