Improved (?) yesod-static TH

Hey!

This pull request adds a form of string interning to `yesod-static`.  I've developed it while trying to find a way of using less than 4 GiB of memory just for my `StaticFiles.hs` module when with optimizations turned on.  However, it seems that it didn't give much of an improvement.  

That being said, it was developed after all.  Instead of just throwing it away, I would like opinions about whether the complexity is worthwhile.