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_aggregate_signature.rs

@@ -9,6 +9,7 @@ use serde::de::{Deserialize, Deserializer};
 use serde::ser::{Serialize, Serializer};
 use ssz::{Decode, Encode};
 use std::fmt;
+use std::hash::{Hash, Hasher};
 use std::marker::PhantomData;
 use tree_hash::TreeHash;
 
@@ -264,6 +265,18 @@ where
     impl_tree_hash!(SIGNATURE_BYTES_LEN);
 }
 
+/// Hashes the `self.serialize()` bytes.
+#[allow(clippy::derive_hash_xor_eq)]
+impl<Pub, AggPub, Sig, AggSig> Hash for GenericAggregateSignature<Pub, AggPub, Sig, AggSig>
+where
+    Sig: TSignature<Pub>,
+    AggSig: TAggregateSignature<Pub, AggPub, Sig>,
+{
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.serialize().hash(state);
+    }
+}
+
 impl<Pub, AggPub, Sig, AggSig> fmt::Display for GenericAggregateSignature<Pub, AggPub, Sig, AggSig>
 where
     Sig: TSignature<Pub>,

crypto/bls/src/generic_signature.rs

@@ -7,6 +7,7 @@ use serde::de::{Deserialize, Deserializer};
 use serde::ser::{Serialize, Serializer};
 use ssz::{Decode, Encode};
 use std::fmt;
+use std::hash::{Hash, Hasher};
 use std::marker::PhantomData;
 use tree_hash::TreeHash;
 
@@ -145,6 +146,13 @@ impl<PublicKey, T: TSignature<PublicKey>> TreeHash for GenericSignature<PublicKe
     impl_tree_hash!(SIGNATURE_BYTES_LEN);
 }
 
+/// Hashes the `self.serialize()` bytes.
+impl<PublicKey, T: TSignature<PublicKey>> Hash for GenericSignature<PublicKey, T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.serialize().hash(state);
+    }
+}
+
 impl<PublicKey, T: TSignature<PublicKey>> fmt::Display for GenericSignature<PublicKey, T> {
     impl_display!();
 }

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

crypto/bls/src/impls/fake_crypto.rs

@@ -113,6 +113,14 @@ impl PartialEq for Signature {
     }
 }
 
+impl Eq for Signature {}
+
+impl std::hash::Hash for Signature {
+    fn hash<H: std::hash::Hasher>(&self, hasher: &mut H) {
+        self.0.hash(hasher);
+    }
+}
+
 #[derive(Clone)]
 pub struct AggregateSignature([u8; SIGNATURE_BYTES_LEN]);