Faster BeaconState enc/dec (#671)

* Add state enc/dec benches

* Add example for flamegraph

* Use `PublicKeyBytes` for `Validator`

* Ripple PublicKeyBytes change through codebase

* Add benches, optimizations to store BeaconState

* Store BeaconState in StorageContainer too

* Optimize StorageContainer with std::mem magic

* Fix rest_api tests

eth2/types/benches/benches.rs

@@ -0,0 +1,79 @@
+use criterion::Criterion;
+use criterion::{black_box, criterion_group, criterion_main, Benchmark};
+use rayon::prelude::*;
+use ssz::{Decode, Encode};
+use types::{
+    test_utils::generate_deterministic_keypair, BeaconState, Eth1Data, EthSpec, Hash256,
+    MainnetEthSpec, Validator,
+};
+
+fn get_state<E: EthSpec>(validator_count: usize) -> BeaconState<E> {
+    let spec = &E::default_spec();
+    let eth1_data = Eth1Data {
+        deposit_root: Hash256::zero(),
+        deposit_count: 0,
+        block_hash: Hash256::zero(),
+    };
+
+    let mut state = BeaconState::new(0, eth1_data, spec);
+
+    for i in 0..validator_count {
+        state.balances.push(i as u64).expect("should add balance");
+    }
+
+    state.validators = (0..validator_count)
+        .into_iter()
+        .collect::<Vec<_>>()
+        .par_iter()
+        .map(|&i| Validator {
+            pubkey: generate_deterministic_keypair(i).pk.into(),
+            withdrawal_credentials: Hash256::from_low_u64_le(i as u64),
+            effective_balance: i as u64,
+            slashed: i % 2 == 0,
+            activation_eligibility_epoch: i.into(),
+            activation_epoch: i.into(),
+            exit_epoch: i.into(),
+            withdrawable_epoch: i.into(),
+        })
+        .collect::<Vec<_>>()
+        .into();
+
+    state
+}
+
+fn all_benches(c: &mut Criterion) {
+    let validator_count = 16_384;
+    let state = get_state::<MainnetEthSpec>(validator_count);
+    let state_bytes = state.as_ssz_bytes();
+
+    c.bench(
+        &format!("{}_validators", validator_count),
+        Benchmark::new("encode/beacon_state", move |b| {
+            b.iter_batched_ref(
+                || state.clone(),
+                |state| black_box(state.as_ssz_bytes()),
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+
+    c.bench(
+        &format!("{}_validators", validator_count),
+        Benchmark::new("decode/beacon_state", move |b| {
+            b.iter_batched_ref(
+                || state_bytes.clone(),
+                |bytes| {
+                    let state: BeaconState<MainnetEthSpec> =
+                        BeaconState::from_ssz_bytes(&bytes).expect("should decode");
+                    black_box(state)
+                },
+                criterion::BatchSize::SmallInput,
+            )
+        })
+        .sample_size(10),
+    );
+}
+
+criterion_group!(benches, all_benches,);
+criterion_main!(benches);