Adds the `TransferOnion` contract which can be used to batch
send tokens. The set of transfers are committed to during
contract initialization, and the token owner must approve
the `TransferOnion` contract and then anybody can unwrap
the commitment to the transfers.

The transfers are committed to with a hash onion. This is
a simple vector commitment that must be linearly unwrapped.
Each hash commits to the next hash and the very first hash
is `bytes32(0)`.