Fix various issues with the non-native slice implementation

[oliverwoodings]

I've been hunting down the source of this bug: https://sentry.qutics.com/sentry/ssg-visitor-engine/issues/58/

I believe I've found it. Our non-native slice implementation is pretty fucked. Right now:

Issues fixed:

• If the array had length 1, it would return an empty array - wat
• The slicing started at the length of the array, instead of the final index of the array
• The slicing stopped before it hit index 0