Proposal: containers: add indexing operations for Set

[Patrick Palka]

I'd like to add the following indexing operations for Data.Set.Set, to 
complement the existing functions for Data.Map.Map:

findIndex :: Ord a => a -> Set a -> Int
lookupIndex :: Ord a => a -> Set a -> Maybe Int
elemAt :: Int -> Set a -> a
deleteAt :: Int -> Set a -> Set a

A tentative patch can be found here 
https://github.com/haskell/containers/pull/13

Discussion period: 2 weeks