Add signed_root to tree_hash crate

eth2/utils/tree_hash_derive/tests/tests.rs

@@ -1,5 +1,5 @@
-use tree_hash::CachedTreeHashSubTree;
-use tree_hash_derive::{CachedTreeHashSubTree, TreeHash};
+use tree_hash::{CachedTreeHashSubTree, SignedRoot, TreeHash};
+use tree_hash_derive::{CachedTreeHashSubTree, SignedRoot, TreeHash};
 
 #[derive(Clone, Debug, TreeHash, CachedTreeHashSubTree)]
 pub struct Inner {
@@ -69,3 +69,32 @@ fn uneven_standard_vs_cached() {
 
     test_standard_and_cached(&original, &modified);
 }
+
+#[derive(Clone, Debug, TreeHash, SignedRoot)]
+pub struct SignedInner {
+    pub a: u64,
+    pub b: u64,
+    pub c: u64,
+    pub d: u64,
+    #[signed_root(skip_hashing)]
+    pub e: u64,
+}
+
+#[test]
+fn signed_root() {
+    let unsigned = Inner {
+        a: 1,
+        b: 2,
+        c: 3,
+        d: 4,
+    };
+    let signed = SignedInner {
+        a: 1,
+        b: 2,
+        c: 3,
+        d: 4,
+        e: 5,
+    };
+
+    assert_eq!(unsigned.tree_hash_root(), signed.signed_root());
+}