Generalise type of deleteBy

The type of `deleteBy` in `Data.List` could easily be generalised in a useful way. ``` deleteBy :: (a -> a -> Bool) -> a -> [a] -> [a] ``` to ``` deleteBy :: (a -> b -> Bool) -> a -> [b] -> [b] ``` There is this closed ticket but no reasons for not generalising. https://ghc.haskell.org/trac/ghc/ticket/3399 Discussion Period: 1 week. Matt