Proposal #2717: Add nubWith, nubOrd

[Yitzchak Gale]

Bart Massey wrote:
>  http://hackage.haskell.org/trac/ghc/ticket/2717

Hmm, this reminds us that Data.Set really needs

insertMember :: Ord a => a -> Set a -> (Bool, Set a)

so that we don't need to traverse the tree twice each
time. And the same for Data.IntSet, of course.
Analogous to existing things like

Data.Set.deleteFindMin
Data.Map.insertLookupWithKey

Hasn't someone made this obvious proposal before?