Adding a #[signed_root(skip_hashing)] macro

Lets the user annotate fields of a struct to skip for signed root
hashing.

Also added tests in a `eth2/utils/tests` crate, so that we can test
whether these derived macros work as intended.

eth2/types/src/attestation.rs

@@ -25,6 +25,7 @@ pub struct Attestation {
     pub aggregation_bitfield: Bitfield,
     pub data: AttestationData,
     pub custody_bitfield: Bitfield,
+    #[signed_root(skip_hashing)]
     pub aggregate_signature: AggregateSignature,
 }
 

eth2/types/src/beacon_block.rs

@@ -27,6 +27,7 @@ pub struct BeaconBlock {
     pub previous_block_root: Hash256,
     pub state_root: Hash256,
     pub body: BeaconBlockBody,
+    #[signed_root(skip_hashing)]
     pub signature: Signature,
 }
 

eth2/types/src/beacon_block_header.rs

@@ -27,6 +27,7 @@ pub struct BeaconBlockHeader {
     pub previous_block_root: Hash256,
     pub state_root: Hash256,
     pub block_body_root: Hash256,
+    #[signed_root(skip_hashing)]
     pub signature: Signature,
 }
 

eth2/types/src/deposit_input.rs

@@ -25,6 +25,7 @@ use test_random_derive::TestRandom;
 pub struct DepositInput {
     pub pubkey: PublicKey,
     pub withdrawal_credentials: Hash256,
+    #[signed_root(skip_hashing)]
     pub proof_of_possession: Signature,
 }
 

eth2/types/src/slashable_attestation.rs

@@ -27,6 +27,7 @@ pub struct SlashableAttestation {
     pub validator_indices: Vec<u64>,
     pub data: AttestationData,
     pub custody_bitfield: Bitfield,
+    #[signed_root(skip_hashing)]
     pub aggregate_signature: AggregateSignature,
 }
 

eth2/types/src/transfer.rs

@@ -32,6 +32,7 @@ pub struct Transfer {
     pub slot: Slot,
     pub pubkey: PublicKey,
     #[derivative(Hash = "ignore")]
+    #[signed_root(skip_hashing)]
     pub signature: Signature,
 }
 

eth2/types/src/voluntary_exit.rs

@@ -24,6 +24,7 @@ use test_random_derive::TestRandom;
 pub struct VoluntaryExit {
     pub epoch: Epoch,
     pub validator_index: u64,
+    #[signed_root(skip_hashing)]
     pub signature: Signature,
 }