Proposal: Add `maxOn` and `minOn` functions to Data.Ord

I think these are useful in general, and that they would make a good addition to `Data.Ord`. The names are inspired by `sortOn`. Definitions: ``` minOn :: Ord b => (a -> b) -> a -> a -> a minOn f x y = case comparing f x y of LT -> x _ -> y maxOn :: Ord b => (a -> b) -> a -> a -> a maxOn f x y = case comparing f x y of GT -> x _ -> y ``` These don't need to be the exact definitions, but they convey the semantics I'm looking for.