Improve List.scanl

[jvoigtlaender]

This has the exact same semantics as the current implementation of List.scanl.

The reason I consider it to be an improvement is the following lines in the current implementation:

     case accAcc of
       ...

       [] ->
         [] -- impossible

If something cannot happen, it's better to use types than code comments to express that. My version has that quality.

(If the current implementation stays, I think it would be more appropriate to replace the line [] -- impossible by a call to Debug.crash.)