Generator aware Folding

I think having Generator adapted fold functions might help make cleaner code in some cases. (The idea is [not new](https://package.elm-lang.org/packages/NoRedInk/elm-random-extra/latest/Random-Extra#fold).)

Signatures like:
```elm
foldl : (a -> b -> Generator b) -> Generator b -> List a -> Generator b
foldr : (a -> b -> Generator b) -> Generator b -> List a -> Generator b
```

and an implementation might look something like (with need for improved naming of variables):

```elm
random_foldl f start list =
let
g : a -> Generator b -> Generator b
g value generator = generator |> Random.andThen (\ack -> f value ack)
in
List.foldl g start list
```

Do you think the signatures make sense? I guess they might be suitable in Random.List module.