Atomic Swap Protocol

[gakonst]

Specify & implement a protocol for N-coin atomic swaps. https://hackmd.io/DgzmJIRjSzCYvl4lUjZXNQ?view# provides some info on this. Probably a multi-step protocol with preimages and reveals needed.

This is a prerequisite for both Plasma Debit and for protocol variants which require fragmentation.

https://ethresear.ch/t/state-channels-and-plasma-cash/1515
https://ethresear.ch/t/plasma-cash-minimal-atomic-swap/3409
https://ethresear.ch/t/plasma-cash-defragmentation/3410
https://ethresear.ch/t/plasma-cash-defragmentation-take-2/3515
https://ethresear.ch/t/plasma-cash-defragmentation-take-3/3737