working test, horrendously ugly but should suffice for now

Conservative types refactor (squashed MaybeKafkaBytes)
Reshuffled tests - having tests.hs be the name makes it easier to know they're the tests from the name of the buffer rather than Main.hs
Maybe-ness is baked into Key and Value based on what you said in the email.

Holding off on any lenses/prisms until time for refactoring of data types and better understanding of batching / error semantics is obtained.