add reward token claims as `vouchers`

[calvbore]

vouchers are stored in the ditto contract as a hash of all the relevant data needed to construct a reward token from an external contract. They are mapped to an address that may claim them. Minting and other details of the reward token are decided by the external contract.

This implementation will allow third parties to create their own interpretation of the reward tokens if they choose.

The composability and modularity of this design may prevent vampire forks as in the case of uniswap v2.

Some design questions to consider:

• Emit an event when a voucher is issued?
• Construct the voucher as a merkle tree rather than a simple hashing of multiple data elements?
• Map to a bool rather than an address, and store the claimant in the voucher hash?
• Is there any other data that should be included in the voucher?

[calvbore]

Need to make sure the voucher tracks if the clone was at the head of the auction que.

[0xbok]

lgtm 🚀