CI: install basic CI + remove clippy linting or make sure the repo passes it

[huitseeker]

At the moment, no PR is likely to pass the CI of this repo, because of :

• a stale rust-toolchain,
• clippy warnings in many places.

We should modify the repo so that it has basic CI. Important note: the clippy lint needless_borrow is often a false positive in this repo, since the verbose references are there to allow the compiler to infer that many functions are in a const context. I'd advise initially #[allow(...)]-ing it.