-- support mulitple generators

    Want to save the state, and later continue the generation.
    In the case of mt19937ar.c, if one saves mt[N] (624 words) and the
    counter mti (1 word), then one can continue the computation by loading
    these values.

    * too much copying...
    * supporting multiple, impure generators is possible though.


-- Also provide a RandomGen instance

-- eager evaluation
    -- fill arrays in chunks
    -- possible api for this?

-- support fast range bounds
    -- too slow on haskell side currently.

-- statistical correctness, DieHard suite.
    -- too complex.

-- coverage tests
    -- fixed in head.

-- move to .Unsafe