Add once_cell to crates available in playground

[sunshowers]

The once_cell crate is super useful to play around with so it would be useful to have this in the playground :)

Please let me know if I did this right! I added once_cell by hand to both the Cargo.toml and to crate-information.json.

[dtolnay]

I believe those files are both generated (by the program in the top-crates directory).

[shepmaster]

We don't currently accept one-off crate additions, but thanks! Please check out https://github.com/integer32llc/rust-playground/blob/master/CRATE_POLICY.md for more details.

[sunshowers]

Thanks! Would you consider:
(a) increasing the number of crates, and/or
(b) making the transitive dependencies of the top crates available as well?

[sunshowers]

(just checked and once_cell is in the top 300)

[dtolnay]

Transitive dependencies of the top crates are available. once_cell gets pulled in now by tokio (top 100) -> signal-hook-registry -> arc-swap -> once_cell.
https://github.com/integer32llc/rust-playground/blob/ab9c098daaeec1fb615f4cf4ec8c355371e20c5b/compiler/base/Cargo.toml#L558-L560

[shepmaster]

Would you consider

Please read the aforementioned policy, which discusses whether transitive dependencies are included (they are) as well as how to discuss changes to the policy. I've written it so that I can stop repeating myself.

[sunshowers]

Apologies, I missed that because it wasn't on the bulleted list. I've submitted #603 with a small rewording -- feel free to accept/decline if it makes sense to you. Thanks.