Refactor, simplify and clean up Engine API

[jeromesimeon]

Several improvements to the Ergo engine API in ergo-engine would provide a more solid foundation for execution on multiple platforms (e.g., Node, WebAssembly, etc).

• [x] Turn engine calls into asyncs again (some platforms such as AssemblyScript execution will require that)
• [x] Replace trigger by invoke main
• [ ] Move some support code currently in the logic manager which should really be part of the runtime, into the engine. Specifically:
  • [x] creation of the top-level invoke call to engine
  • [ ] validation against CTO
• [ ] Remove JIT compilation calls from the engine, focusing execution on compiled code, and removing dependency from the engine to the Ergo compiler.
• [ ] Replace logic manager by something simpler, lower-level and specific to the target execution environment + keep the model manager.