[frontend] Add tweak for Merkle tree hashes (#812)

This PR adds a Keccak256 tweak circuit that will be used to hash the nodes in the XMSS merkle tree for the hash-based signature scheme.

Author: paulcadman

crates/frontend/src/circuits/hash_based_sig/hashing/mod.rs

@@ -2,6 +2,8 @@
 mod base;
 mod chain;
 mod message;
+mod tree;
 
 pub use chain::{CHAIN_TWEAK, FIXED_MESSAGE_OVERHEAD, build_chain_hash, circuit_chain_hash};
 pub use message::{MESSAGE_TWEAK, build_message_hash, circuit_message_hash, hash_message};
+pub use tree::{TREE_MESSAGE_OVERHEAD, TREE_TWEAK, build_tree_hash, circuit_tree_hash};