feat: add keccak256 merkle tree

[alxiong]

This PR:

• introduce macro rules impl_mt_hash_256! for implementing boilerplate for node and digest for MT that has 32 bytes output size
• impl for both Sha3 and Keccak256 under this macro
• expose a MT and lightweight MT for keccak256

@mrain do you remember why we pick rate 3 for MT? is that an assumption for our current MT internals?

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