Consider simplifying `fold`

In addition to a list and an anonymous function,`fold` also accepts an initial value.

Example:
``` kari
1 [ 2 3 ] [ + ] fold # evaluates to `6`
```

It would be simpler to drop the first argument:
``` kari
[ 2 3 ] [ + ] fold # evaluates to `5`
```

If an initial value is needed, it can be easily added to the list:
``` kari
[ 2 3 ] 1 prepend [ + ] fold # evaluates to `6`
```