Use Reuse 4 to manage licenses

[Kobzol]

This PR introduces REUSE.toml files for managing licenses and copyright in this repository, including all the compile-time benchmarks under the collector/compile-benchmarks directory. More context can be found in #1939.

For the existing compile-time benchmarks, I used this "algorithm":

1. If it was an artificial stress test created by rustc developers, I used the copyright The Rust Project Developers (see https://thanks.rust-lang.org) and the MIT license. Note that we couldn't use MIT OR Apache-2.0, because the compile-benchmarks directory was only licensed under MIT previously.
2. If it was a third-party crate, I took its licenses (they were present for all the crates, luckily). As for copyright, if there was an explicit copyright in the README, I used it (this happened I think only for one crate). Otherwise, I tried to look for "XYZ developers" or something like that in the authors field of the crate's Cargo.toml. If I haven't found that, I just used <crate-name> contributors.

Fixes: #1939

[Kobzol]

I'm not sure if it's fine to land 6a2cba8, I don't know if e.g. GitHub recognizes REUSE.toml.

[pietroalbini]

Looks good to me! Left one optional nit.

[Kobzol]

Thanks! Let's see what happens on the repo's GH homepage once the old licenses are removed.