Add crates.io benchmark

[Kobzol]

Opening as a draft, as an example of a real-world async application.

[nnethercote]

I found similar things to the zulip discussion:

• 600+ dependencies, that's a lot, it takes a long time to compile them all. I even had some trouble with my 16GB /tmp partition filling up and causing multi-profile+multi-scenario runs to fail, something I've never experienced before.
• And after all that, the final binary is rather small. ~0.5s for a check full build, ~2s for an opt full build.
• The Cachegrind profile is not that exciting. It has some of the new metadata stuff we got in large-workspace, but in smaller proportions. Apart from that, the rest of the profile looks similar to many others.

Overall it doesn't seem like it's too expensive, and not valuable enough.

[Kobzol]

Yeah the binary probably wasn't a good choice. I can try to switch this to the library instead later.