some minor internal renaming in tests
adds benchmarks to root aggregate and forks them in run
internally includes a field which carries the amount of data fed to the checksum

- completely invisible to the end user, negligible perfomance impact
- makes the monoid implementations much simpler/nicer
- makes the scalaz monoids faster (don't know why exactly,
  must be some JVM optimization thingy)

- also more benchmarks concerning the idiomatic use cases for the monoids
  (foldLeft, foldMap, suml, parallel collections)

- the specs and the benchmarks now work with foldMap
- I guess it is the usage of foldable instead of traverse