[experiment] Remove #[handle_result]

[uint]

Pushing this early since I'm stupidly excited and figured it's interesting (:

Once I have a PoC, I expect this can be expanded into an ergonomic API for:

• structured errors with schemas (note there's already a way to choose the serialization format)
• opt-out rollback
• opt-in Result serialization (if asked for?)

[PolyProgrammist]

@uint hi, I am working on #936 and wondering what are the results of experiment?