std: using HashSet causes write permission fault

[DemesneGH]

In this PR #209 (comment), using std::collections::HashSet results in a TA panic with the write permission fault when running the secure_db_abstraction example.

Replacing it with hashbrown::HashSet resolves the issue, suggesting there may be a problem in our current std implementation of HashSet.

Marking this as a TODO for further investigation.