Integrate Parsley into testsuite / head.hackage somehow?

I just watched Nick Wu and Jamie Willis's talk on Staged Selective Parser Combinators. The implementation, https://github.com/J-mie6/ParsleyHaskell, uses Typed Template Haskell, levity polymorphism, heterogeneous lists, and likely other pyrotechnics. We should use it as a benchmark for GHC -- including a performance one, given that the runtime performance of their parser library is one of their goals.