Add benchmarks for tree_hash_root

Cargo.toml

@@ -30,3 +30,7 @@ criterion = "0.3"
 [[bench]]
 name = "rebase"
 harness = false
+
+[[bench]]
+name = "tree_hash_root"
+harness = false

benches/tree_hash_root.rs

@@ -0,0 +1,49 @@
+use criterion::{criterion_group, criterion_main, BenchmarkId, Criterion};
+use milhouse::{List, Vector};
+use ssz_types::VariableList;
+use tree_hash::TreeHash;
+
+type C = typenum::U1099511627776;
+type D = typenum::U1000000;
+const N: u64 = 1_000_000;
+
+pub fn tree_hash_root(c: &mut Criterion) {
+    let size = N;
+
+    c.bench_with_input(
+        BenchmarkId::new("tree_hash_root_list", size),
+        &size,
+        |b, &size| {
+            b.iter(|| {
+                let l1 = List::<u64, C>::try_from_iter(0..size).unwrap();
+                l1.tree_hash_root()
+            });
+        },
+    );
+
+    c.bench_with_input(
+        BenchmarkId::new("tree_hash_root_vector", size),
+        &size,
+        |b, &size| {
+            b.iter(|| {
+                let v1 = Vector::<u64, D>::try_from_iter(0..size).unwrap();
+                v1.tree_hash_root()
+            });
+        },
+    );
+
+    // Test `VariableList` as a point of comparison.
+    c.bench_with_input(
+        BenchmarkId::new("tree_hash_root_ssz_types_list", size),
+        &size,
+        |b, &size| {
+            b.iter(|| {
+                let l1 = VariableList::<u64, C>::new((0..size).collect()).unwrap();
+                l1.tree_hash_root()
+            })
+        },
+    );
+}
+
+criterion_group!(benches, tree_hash_root);
+criterion_main!(benches);