[Simplicity] Smart Contracts Unchained

[ZmnSCPxj]

https://zmnscpxj.github.io/bitcoin/unchained.html

Smart contracts have traditionally been implemented as part of the consensus rules of some blokchain.  Often this means creating a new blockchain, or at least a sidechain to an existing blockchain.  This writeup proposes an alternative method without launching a separate blockchain or sidechain, while achieving security similar to federated sidechains and additional benefits to privacy and smart-contract-patching.