MerkleProof rebuild root.

[Amxx]

Sometimes it can be usefull to rebuild the root given a leaf and proof, but not check it matches a known value.

For example, one might have a mapping of valid roots mapping(bytes32 => bool) private _valid and may want to run

require(_valid[MerkleProof.getRoot(leaf, proof)], "proof doesn't match a valid merkle tree");