Expose method for initializing new StdGen from SplitMix.initSMGen

[Mathnerd314]

From #69 it seems that the preferred method for using the library is something like

main = do
  s <- round <$> getPOSIXTime
  g <- newIOGenM $ mkStdGen s
  -- whatever thing using randomness
  l <- replicateM 10 $ uniformRM (1 :: Int, 100) g
  print l

Using POSIXTime seems hacky, when there is a nicer method of getting a random seed:

import Control.Monad
import System.Random
import System.Random.Stateful
import System.Random.Internal
import qualified System.Random.SplitMix as SM

initStdGen :: IO StdGen
initStdGen = StdGen <$> SM.initSMGen

main = do
  g <- initStdGen >>= newIOGenM
  -- whatever thing using randomness
  l <- replicateM 10 $ uniformRM (1 :: Int, 100) g
  print l

But it isn't a good idea to import internal modules, so initStdGen has to be added to the API.

[lehins]

This is a perfectly sensible request IMHO.