Restrict type of monadic binding

According to Do notation considered harmful#Safety the type of the monadic binding combinator (>>) should be restricted to

(>>) :: m () -> m a -> m a

This way, you can omit _ <- only if the monadic return value has type ().

Types of some other monadic operations should be restricted, too.

mapM_ :: Monad m => (a -> m ()) -> [a] -> m ()
replicateM_ :: Monad m => Int -> m () -> m ()
sequence_ :: Monad m => [m ()] -> m ()
zipWithM_ :: Monad m => (a -> b -> m ()) -> [a] -> [b] -> m ()