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

[Daniel Díaz Casanueva]

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.
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