Add `leaf_hash` function to `SparseMerkleTree`

[antazoey]

What feature should we add?

The equivalent of https://github.com/OpenZeppelin/merkle-tree/blob/master/src/standard.ts#L114-L116

[antazoey]

It looks like the data is abi encoded before keccaked: https://github.com/OpenZeppelin/merkle-tree/blob/master/src/utils/standard-leaf-hash.ts#L7