Using CompactArray for permutations

[progval]

speaking of space used for permutations, I just noticed that we could probably halve it in the Software Heritage graph by using CompactArray instead of naive arrays, as we have 34 billion nodes (2^35).

Do you want me to try and see the impact on LLP performance?

[vigna]

Well, the only difference should be in the generation and inversion of the permutation. The problem is that CompactArray in the atomic version does not have very strong concurrency guarantees, so you wouldn't be able to use parallel sorting when generating it.