use SHA512 when seeding MersenneTwister
This hashes the provided seeds before feeding dSFMT initialization routine.
The passed seeds are still stored as-is in the `MersenneTwister.seed` field.
Goals:
* possibly lead to more independance between two streams corresponding to two
user-provided seeds
* by feeding `VERSION.minor` and `VERSION.major` to the hashing process, this
guarantees that random streams change between minor releases, helping users
to not believe in stream stability
