Draft: Improve MonadZip instances

• Use fmap instead of liftM for the default method definitions.
• Make munzip stricter for most types to avoid space leaks and to conform better to intuition from Data.List.unzip.
• Define munzip for transforming types use the underlying munzip rather than the default, which ignores that.

I've made a number of judgement calls about desired laziness based on my own intuition. I hope those match others' understanding