Function for mapping both error and ok

```elm
mapBoth : (Result e a -> Result e1 a1) -> Script e a -> Script e1 a1
mapBoth map =
Script.attempt
>> Script.thenWith
(\result ->
case map result of
Ok ok ->
Script.succeed ok

Err err ->
Script.fail err
)
```
It would be nice to have a function like this as it's not entirely obvious how to implement it with the existing functions and it's useful to have this level of flexibility sometimes.