draft: use generics in Converter

[mattxwang]

Trying to fix issues with sorbet-coerce and tapioca :(

[mattxwang]

@katyho I think I forgot to write up the problem this PR was trying to solve (or ... it's buried in a Google doc I don't have access to).

For completion's sake, I'll submit an issue and/or try to resolve the problem myself sometime soon:tm:! Apologies on not wrapping this up before I finished 😓

(also ... I think I do have an open PR on traject that's functionally good to merge, but needed some approval from Jackie. Feel free to do whatever you think makes sense for that PR!)

[mattxwang]

Okay - have written up (maybe my last 😓 ) super-long brain dump in #77 - let me know if that makes sense @katyho!

I'll admit: since I don't have the original codebase that caused this error, I could be wrong in the context I explained it in. Hopefully it's simple to repro (delete my shim in infra-tasks, re-enable tapioca codegen for sorbet-coerce, and then run tapioca gem sorbet-coerce and see a failed static typecheck).