Prepare for attoparsec 0.12

I'm about to release attoparsec 0.12, which improves the time performance of cassava parsers by roughly 10%, while reducing space usage by about 33%.

You'll need to apply the following patches to cassava to get it to work with attoparsec 0.12. These changes are compatible with all the attoparsec releases that cassava works with today.

It would be good to release a new cassava that will work with the new attoparsec at your convenience.

Thanks!