On chain merkle proof library

[dob]

We need merkle proof verification for #24. Ideally this is implemented as a library that's reusable not just in Livepeer, but elsewhere as well. Perhaps we can open as an issue and PR into OpenZeppelin.

Yondon also found this, which may be a good starting point: https://github.com/ameensol/merkle-tree-solidity/blob/master/src/MerkleProof.sol

[yondonfu]

In progress here: OpenZeppelin/openzeppelin-contracts#260

[dob]

Also done?

[yondonfu]

Just got merged into OpenZeppelin and should be included in the next release. Will update the protocol contracts to use the OpenZeppelin release when that release is out