Add Keccak hasher (#6101)

* fix keccak hasher

* Update hasher.rs

Co-authored-by: Gavin Wood <i@gavwood.com>

substrate/primitives/core/src/hasher.rs

@@ -36,3 +36,23 @@ pub mod blake2 {
 		}
 	}
 }
+
+pub mod keccak {
+	use hash_db::Hasher;
+	use hash256_std_hasher::Hash256StdHasher;
+	use crate::hash::H256;
+
+	/// Concrete implementation of Hasher using Keccak 256-bit hashes
+	#[derive(Debug)]
+	pub struct KeccakHasher;
+
+	impl Hasher for KeccakHasher {
+		type Out = H256;
+		type StdHasher = Hash256StdHasher;
+		const LENGTH: usize = 32;
+
+		fn hash(x: &[u8]) -> Self::Out {
+			crate::hashing::keccak_256(x).into()
+		}
+	}
+}