Add minIndex and maxIndex SCL functions into Prelude

minIndex :: Ord a => IndexedSequence b => Sequence (b a) => b a -> Integer
minIndex xs = minimumBy (xs!) [0..length xs -1]

maxIndex :: Ord a => IndexedSequence b => Sequence (b a) => b a -> Integer
maxIndex xs = maximumBy (xs!) [0..length xs -1]