map/filter with foldr

第七课的小练习：http://www.cs.nott.ac.uk/~gmh/chapter7.ppt

> map' :: (a -> b) -> [a] -> [b]
> map' f = foldr (\a b -> (f a) : b) []
>
> filter' :: (a -> Bool) -> [a] -> [a]
> filter' f = foldr f' [] where
>     f' a b = if f a then a:b else b