#2309: containers: specialize functions that fail in a Monad to Maybe

[Ross Paterson]

Several functions on containers used to have types like

	lookup :: (Ord k) => k -> Map k a -> Maybe a

but these were generalized to

	lookup :: (Monad m, Ord k) => k -> Map k a -> m a

The only methods of Monad used are return and fail. The problem is that,
depending on the monad, fail can be an ordinary value or a runtime error:
this device makes it harder to check whether a program is safe, because
it hides possible runtime errors among testable conditions.

The proposal is to change these signatures back, specializing them
to Maybe.

The functions involved are:

	lookup :: Ord k => k -> Map k a -> Maybe a
	lookupIndex :: Ord k => k -> Map k a -> Maybe Int
	minViewWithKey :: Map k a -> Maybe ((k,a), Map k a)
	maxViewWithKey :: Map k a -> Maybe ((k,a), Map k a)
	minView :: Map k a -> Maybe (a, Map k a)
	maxView :: Map k a -> Maybe (a, Map k a)

	lookup :: Key -> IntMap a -> Maybe a
	maxViewWithKey :: IntMap a -> Maybe ((Key, a), IntMap a)
	minViewWithKey :: IntMap a -> Maybe ((Key, a), IntMap a)
	maxView :: IntMap a -> Maybe (a, IntMap a)
	minView :: IntMap a -> Maybe (a, IntMap a)

	minView :: Set a -> Maybe (a, Set a)
	maxView :: Set a -> Maybe (a, Set a)

	maxView :: IntSet -> Maybe (Int, IntSet)
	minView :: IntSet -> Maybe (Int, IntSet)

No information is lost, because in each case there is a single failure
mode.