fix: use keccak for leaves.

Author: shotaronowhere

contracts/src/bridge/merkle/MerkleProof.sol

@@ -39,7 +39,7 @@ contract MerkleProof {
         bytes memory data,
         bytes32 merkleRoot
     ) public pure returns (bool) {
-        return validateProof(proof, sha256(data), merkleRoot);
+        return validateProof(proof, keccak256(data), merkleRoot);
     }
 
     /** @dev Calculates merkle root from proof.

contracts/src/bridge/merkle/MerkleTreeHistory.sol

@@ -40,7 +40,7 @@ contract MerkleTreeHistory {
      *  @param data The data to insert in the merkle tree.
      */
     function append(bytes memory data) public {
-        bytes32 leaf = sha256(data);
+        bytes32 leaf = keccak256(data);
         count += 1;
         uint256 size = count;
         uint256 hashBitField = (size ^ (size - 1)) & size;