Random functor monad

I added functions

> map : (a -> b) -> Generator a -> Generator b

and

> andThen : Generator a -> (a -> Generator b) -> Generator b

to the `Random` module. Is there another name you prefer to `andThen`? I was shooting for consistency with the `Maybe` module. Also, does anyone have any ideas about a uniform convention for Applicative "instances"? Currently `Random` exports `pair : Generator a -> Generator b -> Generator (a, b)`, but as far as I can tell no other module exports a function of the same name.