Lazy hashing for SignedBeaconBlock in sync (#2916)

## Proposed Changes

Allocate less memory in sync by hashing the `SignedBeaconBlock`s in a batch directly, rather than going via SSZ bytes.

Credit to @paulhauner for finding this source of temporary allocations.

crypto/bls/src/generic_signature_bytes.rs

@@ -9,6 +9,7 @@ use serde::ser::{Serialize, Serializer};
 use ssz::{Decode, Encode};
 use std::convert::TryInto;
 use std::fmt;
+use std::hash::{Hash, Hasher};
 use std::marker::PhantomData;
 use tree_hash::TreeHash;
 
@@ -84,6 +85,12 @@ impl<Pub, Sig> PartialEq for GenericSignatureBytes<Pub, Sig> {
     }
 }
 
+impl<Pub, Sig> Hash for GenericSignatureBytes<Pub, Sig> {
+    fn hash<H: Hasher>(&self, hasher: &mut H) {
+        self.bytes.hash(hasher);
+    }
+}
+
 /// Serializes the `GenericSignature` in compressed form, storing the bytes in the newly created `Self`.
 impl<Pub, Sig> From<GenericSignature<Pub, Sig>> for GenericSignatureBytes<Pub, Sig>
 where