Updated cabal file with -O2 options

Added -O2 to the GHC build options for the library and the benchmarks. Is there any reason not to have this there already? The -O/-O2 difference seems to matter in these cases -- I just spotted a little performance improvement when adding the -O2 option to cereal (it doesn't have -O2 for now) for a cereal/binary benchmark, so I'm just wondering if there's a reason why it's not there in your cabal file yet.