Fix slices by cheating with recast

[GoodForOneFare]

Fixes:

• Off-by-1 errors for inclusive ranges
• Ensure ranges ending with a var never evaluate to 0

I started out by trying to fix this without leaning on recast, but the code gets ugly once you have to consider different node types for positive/negative end bounds. I gave up and used recast once the variable-end-bound-should-never-be-0 thing cropped up :/

[lemonmade]

Ping @juliankrispel can you take a look at this PR and the others Gord put up?

[juliankrispel]

@lemonmade @GoodForOneFare sry guys, my notifications here must be screwed up