Add -O2 to the ghc build options

I just saw the cabal file doesn't include -O2 in the options passed to GHC when building the library. Is there any reason for this? 

There are some key optimizations that are performed only with -O2 AFAIK, and they can make a difference on cereal's code maybe? 

I seem to gain consistently a few dozens of microsecs on the benchmarks, in particular it becomes significantly faster than 'binary' (which is built with -O2) on the last two benchmarks.