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).

• Turn engine calls into asyncs again (some platforms such as AssemblyScript execution will require that)
• 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:
  • 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.