Move Ordered Merkle Tree to Open Zeppelin

[hswick]

Right now it uses a linked list approach, need a more complex data structure for better performance

[hswick]

@mrsmkl has these little util functions that should be used here:

   function makeMerkle(bytes arr, uint idx, uint level) internal pure returns (bytes32) {
      if (level == 0) return idx < arr.length ? bytes32(uint(arr[idx])) : bytes32(0);
      else return keccak256(makeMerkle(arr, idx, level-1), makeMerkle(arr, idx+(2**(level-1)), level-1));
   }

   function calcMerkle(bytes32[] arr, uint idx, uint level) internal returns (bytes32) {
      if (level == 0) return idx < arr.length ? arr[idx] : bytes32(0);
      else return keccak256(calcMerkle(arr, idx, level-1), calcMerkle(arr, idx+(2**(level-1)), level-1));
}

[hswick]

https://github.com/ameensol/merkle-tree-solidity seems like the way to go

[hswick]

Merged a rudimentary integration for now. I think long term it would be best to move this code to a more permanent location where Truebit is not responsible for maintaining it like say the open-zeppelin repos.

The code was taken from here and modified it a bit to work with this code for now.

Like I said above, I think its final form would be to add the Ordered Merkle Tree features to this repo (see relevant code here)

This way we can simply import the code as a dependency