Proposal: add fusion RULES for Data.Map.mapMaybe

Hi, Currently mapMaybe does not fuse at all. The attached patch implements necessary rules for fold/build fusion in both sides (the result and the second argument) to happen. When fusion does not happen, the function should behave exactly the same as before. I ran some benchmarks, testing cases where fusion does and does not happen: http://htmlpreview.github.io/?https://github.com/takano-akio/mapmaybe-benchm... The benchmark code is: https://github.com/takano-akio/mapmaybe-benchmarks/blob/master/main.hs Discussion period: 2 weeks. Thank you, Takano Akio