mfp

RList.make_from_s cannot be used within update step

2

`make_from_s` [uses `React.S.value`](https://github.com/hhugo/reactiveData/blob/a1a6c6523a596b91eb6062204fba2eff3273bc1a/src/reactiveData.ml#L145), which [can raise a `Failure` when executed within an update step](http://erratique.ch/software/react/doc/React.S.html#VALvalue). I haven't been able to define it using only React non-primitive operators. Even though [coping out...