[refactor] Inference engines

[paxcema]

Changelog:

• Reorganized codebase into inference engines
• Current approach is now the "rule based" engine
• Define new class abstractions
• Move existing engine from method to class
• Figure out if the current fan out approach is general enough for other engines.
• Clean up API entry points
• Refactor so that there are approach-specific reqs
• Organize poetry deps into core + per-engine

[pedrofluxa]

Looks good and tidy to me. I think this refactor is a great idea that will help maintain and extend our current type inference capabilities.

Approving PR.