Using nightly one leads to having to resolve lints more frequently than rust releases, which is
getting inconvenient.
It also leads to a small compatibility issue with stable clippy: if we need to #[allow] a lint that
is only present in nightly clippy, then stable clippy complains about unknown ling in the `allow`.
* Tidy code a bit thanks to clippy
Clippy 1.54 newly detects some redundant constructs, that's nice.
sort_unstable() should yield exact same results as sort() for `Vec<&str>`
and could be faster, clippy says.
* Add clippy to CI
Matěj Laitl