pointed array comonad

Hi Edward, 
I have had this array comonad definition floating around that I use a lot. I decided to add it to  a fork of your package since I'm always using it. Not sure if it's something you want to include (having more instances of Comonad), but thought I'd do a pull request to let you decide- I won't be offended if you don't want to include it! There are of course other instances that could be provided for other Haskell data types which I may add at some point (maybe for Tree, but perhaps this should go straight into the existing containers package). 

Cheers,
Dominic