Revisit negative array indexing

[ricardbejarano]

So while this was discussed in #113 back in 2016, I wonder if we should revisit this discussion given the recent changes to array slices in #1093.

With array slicing supporting negative indexes, I find it counterintuitive not to have negative indexing in regular array lookups. Especially as we seem to approximate Python's slicing, and Python does have negative indexing.

@sparkprime you made a good argument against it here, I'm curious if you've changed your mind as hinted here.

Note: I've had this related PR in go-jsonnet open for a while.