Column openings in linear-code-based PCSs

[Antonio95]

This issue is related to Arkworks' poly-commit library and not our codebase, but since we make crucial use of those PCSs here, I open this issue as a reminder.

As @mmagician noticed, the indices of columns opened in linear-code-based PCSs (Brakedown, Ligero) are not checked to be unique, potentially compromising (at least formally - and perhaps practically in the case of Brakedown) soundness. The fix should be relatively easy for a potentially non-negligible performance costs (searching each generated index in the list of previously generated ones). Although a separate discussion, performance would likely take a major hit in recursive settings.

@mmagician Let me know if you would prefer me to remove this issue from this repo, open it in Arkworks, both or neither :)

[mmagician]

for now let's keep it here private. If we have time figure this out properly with an analysis, we should do a responsible bug disclosure, since there are more libs that are affected.