for loop should *maybe* return `null`

This is R's behavior:

```r
> x <- for(i in 1:10) i
> x
NULL
```

This the behavior in the re-implementation:

```r
> x <- for (i in 1:10) i
[1] 10
> x
[1] 10
>
```

I guess nobody uses the return values of for loops anyway, but I think it is more intuitive for *for loops* to return `null` instead of the last evaluation in the last iteration.