Cats-effect 3.x roadmap

[Daenyth]

I'll roll out updates for cats-effect 3.x in the following manner:

• On CE2, redesign the api to work with a Converter[F] type rather than globally-available implicit methods. #84
• Update to CE3 using that new api.

This should help ease the migration pain, since folks can tackle the big api change on CE2 before upgrading the rest of their app