Soundness issues in dependency `sized-chunks` (via `im` crate)

[GeorgeHahn]

See bodil/sized-chunks#11.

[dbrgn]

This also has a rustsec advisory: https://rustsec.org/advisories/RUSTSEC-2020-0041

Note that unfortunately there are no fixes so far.

[jan-auer]

This also relates to #257, suggesting that we run the audit action as part of CI.

On a more direct note, this will require some closer investigation, as this is still open in sized-chunks and we rely heavily on im.

[Roguelazer]

Any update here? It's been three months and every project that uses sentry has unfixable rustsec advisories against it, which kind of unfortunate? The fact that the advisory causes memory safety issues during a panic is particularly unfortunate since sentry pretty often is involved in panics...

[Swatinem]

Like @jan-auer said, creating new hubs / pushing/popping scopes relies heavily on im, and it would seem that that library has been abandoned.
Which is unfortunate. We can just kick it our and use Arc and make_mut everywhere, at the cost of more allocations probably, so it is doable.

[dbrgn]

Thanks @Swatinem!