Add props and test cases using mapK and mapV

May have found a mapK bug, it seems that:

runMapSyntax M.lookup M.insert $ do { foo ; mapK (++"a") bar} == runMapSyntax M.lookup M.insert $ do { mapK (++"a" bar) }

(looking into that now).  

For another example, try this: https://gist.github.com/ImAlsoGreg/98092ade3a086b14d356