Tree hash benches (#486)

* Add initial tree hash benches

* Add tree hash example

* Use lazy static in tree hash benches

eth2/utils/tree_hash/examples/flamegraph_beacon_state.rs

@@ -0,0 +1,35 @@
+use tree_hash::TreeHash;
+use types::test_utils::TestingBeaconStateBuilder;
+use types::{BeaconState, EthSpec, MainnetEthSpec};
+
+const TREE_HASH_LOOPS: usize = 1_000;
+const VALIDATOR_COUNT: usize = 1_000;
+
+fn build_state<T: EthSpec>(validator_count: usize) -> BeaconState<T> {
+    let (state, _keypairs) = TestingBeaconStateBuilder::from_default_keypairs_file_if_exists(
+        validator_count,
+        &T::default_spec(),
+    )
+    .build();
+
+    assert_eq!(state.validators.len(), validator_count);
+    assert_eq!(state.balances.len(), validator_count);
+    assert!(state.previous_epoch_attestations.is_empty());
+    assert!(state.current_epoch_attestations.is_empty());
+    assert!(state.eth1_data_votes.is_empty());
+    assert!(state.historical_roots.is_empty());
+
+    state
+}
+
+fn main() {
+    let state = build_state::<MainnetEthSpec>(VALIDATOR_COUNT);
+
+    // This vec is an attempt to ensure the compiler doesn't optimize-out the hashing.
+    let mut vec = Vec::with_capacity(TREE_HASH_LOOPS);
+
+    for _ in 0..TREE_HASH_LOOPS {
+        let root = state.tree_hash_root();
+        vec.push(root[0]);
+    }
+}