Propagate more compile benchmark metadata into the dashboard

[Kobzol]

This PR adds a new required field to perf-config.json, artifact. It has to be either library or binary, it specifies which target does the benchmark build. There's now a unit test that checks that all perf-config.json contain this attribute and that they be correctly parsed. Before this would only be found out during actual benchmarking or when displaying the compare page.

Furthermore, information about compile-time benchmarks that is useful (e.g. did the benchmark override LTO/CGU/debug information? does it build a binary or a library?) is now propagated into the compare page dashboard.

The information is extracted from the compile benchmark directory (and its perf-config.json and Cargo.toml files) via a build script of the site package. It is then sent to the compage page via the API and displayed as a simple tooltip over each benchmark name in the comparison table. The build script takes ~0.05s to execute, and is cached unless something in collector/compile-benchmarks changes (it seems that the detection whether something has changed is quite fast and doesn't slow down site rebuilds).

In addition, the filters can now hide/show all binaries/all libraries. This filtering is implemented using the newly added metadata.

Fixes: #1619

[lqd]

This LGTM but I'm not familiar with:

• some parts of the collector/api
• using serde in real life, and the intricacies of missing data, backwards compatibility in format, deserialization errors, etc

so maybe another quick look from someone more familiar with these could be worthwhile

[Kobzol]

The frontend side was reviewed by @lqd, also requesting review from @Mark-Simulacrum about the config changes and the propagation of data into site.

[nnethercote]

Do these instructions need updating?

[Kobzol]

Good point! I updated it, along with the glossary.