skip and mix methods in Random typeclass?

There are RNGs that can skip n values in less than O(n) time, and they should have the opportunity to expose this to the user. And the ones that don't still have a sensible default:

``` haskell
skip :: (RandomGen g) => Int -> g -> g
skip n g | n <= 0 = g
skip n g = case next g of (_, g') -> skip (n - 1) g'
```

There are also RNGs that allow the user to mix in entropy to the generator, and this also has a sensible default for generators without that operation:

``` haskell
pick :: Bool -> (a, a) -> a
pick False (f, s) = f
pick True (f, s) = s

mix :: (RandomGen g) => Int -> g -> g
mix = go (finiteBitSize (0 :: Int)) where
go n k g | n `seq` k `seq` g `seq` False = undefined
go n k g | n < 0 = g
go n k g = go (n - 1) k (testBit K n `pick` split g)
```

These methods should be part of the typeclass so they can be available to both consumers of random numbers and those who want to create generators that modify other generators.