fromListWith is confusing

[sjakobi]

This is rather surprising:

> fromListWith (++) [(False,"x"),(False,"y")]
fromList [(False,"yx")]

"Intuitively", the value in the result should be "xy".

Note that the versions of fromListWith[Key] in Data.Map and Data.IntMap from containers have the same issue.

[sjakobi]

@treeowl suggested less ambiguous types for versions of fromListWithKey in #246 (comment):

flwk
  :: (Hashable k, Eq k)
  => (k -> Maybe b -> a -> Box b) -> [(k,a)] -> HashMap k b

(where data Box a = Box a)

flwkm
  :: (Hashable k, Eq k)
  => (k -> Maybe b -> a -> Maybe b) -> [(k,a)] -> HashMap k b

[sjakobi]

FWIW, there's another issue about fromListWith, but that one seems to be primarily concerned with performance: #162