Foldable/Traversable instances

[noinia]

Some comments:

• I've used qualified imports to import Foldable/Traversable as to be compatible with older GHC's. I'm not sure what the policy is here (and/if you want to guard these imports with CPP to avoid warnings in recent GHC's).
• Traversable should traverse the list from left to right. I think the most natural definition starts with at the focus (which is what I've done here). I've added a comment to indicate this.

[jeremyjh]

Thanks, I've merged and uploaded to Hackage.

[noinia]

Thanks! Although I think I've screwed up something :(. I think I should have reversed the left list of the zipper (it now traverses that part from right to left instead of left to right). The right thing to do should be:

traverse g (CList l f r) = (\f' r' l' -> CList (reverse l') f' r') 
                        <$> g f
                        <*> T.traverse g r
                        <*> T.traverse g (reverse l)

Initially, I wrote traverse as:

traverse g = fmap fromList . traverse g . rightElements 

which is cleaner, however it potentially changes the internal structure of the CList. This is not observable from the outside though. Which option do you think we should use?