STAR and BY in list slices

This commit enables full use of the STAR (`*`) and BY keyword in list slices. It seems a bit arbitrary to allow `*` in list indexes but not in slices, as it is today. Similarly, `list[X to Y by Z]` only works in the current build if both X, Y and Z are integer literals, which is a bit confusing.

__Examples:__
Grab tail of list: `list[1 to *]`
Grab last half of list: `list[*/2 to *]`
Grab every second item: `list[to * by 2]`
Reverse list: `list[* to 0 by -1]`
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Disclaimer: I am unfamiliar with the compiler and have no idea if my solution is the most elegant way of solving this.