Fix takeWhile :: (a -> Bool) -> [a] -> [a]

The prior definition of takeWhile and dropWhile only check the first element.

> takeWhile even [2,3,4,5,6]
[2]
> dropWhile even [2,3,4,5,6]
[3,4,5,6]

where it should be:

>takeWhile even [2,3,4,5,6]
[2,4,6]
>dropWhile even [2,3,4,5,6]
[3,5]