Proof of concept / WIP.
Split ERC20s into promise tokens that can be redeemed after the merge.
Before the merge, deposit one XXX and receive two "promise tokens": one XXXW and one XXXS.
Before the merge, burn one XXXW and one XXXS to redeem one XXX.
After the merge, on PoS Ethereum, burn one XXXS to redeem one XXX.
After the merge, on PoW Ethereum, burn one XXXW to redeem one XXX.
Before and after the merge, XXXW and XXXS can be transferred freely.