Allow OverloadedLists to overload (:) in patterns and expressions

@simonpj suggested (here) that the OverloadedLists extension should also overload the (:) constructor, both in expressions and patterns. This requires adding new methods to the IsList typeclass; Simon suggests:

class IsList l where
  type Item l

  fromList :: [Item l] -> l
  toList   :: l -> [Item l]

  fromListN :: Int -> [Item l] -> l
  fromListN _ = fromList

  cons   :: Item l -> l -> l
  unCons :: l -> Maybe (Item l, l)

This will allow users to write code that looks like it should work with overloaded lists, such as:

f [] = 1
f [_] = 2
f (x:xs) = x

Simon notes that this will probably require a GHC proposal.