Proposal: add fusion RULES for Data.Map.mapMaybe

[Akio Takano]

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-benchmarks/blob/master/out.html

The benchmark code is:

https://github.com/takano-akio/mapmaybe-benchmarks/blob/master/main.hs

Discussion period: 2 weeks.

Thank you,
Takano Akio
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