`sharded-slab` dependency appears to be unmaintained

[bal-e]

I noticed this when investigating why an old version of lazy_static was getting pulled into our dependency tree. It appears that sharded-slab, used by tracing-subscriber, hasn't been updated since Oct 2023 (only a few minor commits since then, and no releases). There are a number of open PRs that the maintainer hasn't responded to. Perhaps moving to a better-maintained library would be a good idea?

I see that slab is a similar crate, only offering single-threaded use, but it is maintained by the Tokio team. Perhaps enhancing it with concurrent access would be feasible.

(Personally, I quite like building these kinds of concurrent data structures; but I'm completely swamped at the moment.)

[davidbarsky]

tracing used to use the slab crate, but it wasn't particularly scalable across multiple cores. We should probably get a release of sharded-slab out, though.