Placeholder for Capers: Stateless Coprocesses

They use the same FANOS protocol as Headless Mode

• OLD documents from 2018, before we implemented descriptor passing:
  • Coprocess Protocol Proposal / Coprocess Protocol V2 / Coprocess Language Support
  • old code: https://github.com/oilshell/shell-protocols/tree/master/coprocess
    • our demo of Capers can use the same 200 ms startup time. Or we can do a more realistic example with HTTP.