Nix flake & CI

[wsx-antithesis]

Setup Nix flake & CI to test:

• The project builds with the latest nightly and the stable MSRV (declared in lib/Cargo.toml) Rust compiler.
• cargo clippy --all-targets -- -D warnings passes.
• cargo test passes.
• cargo doc builds.
• The Rust files are formatted with rustfmt, and Nix files formatted with nixpkgs-fmt.
• All the above but after cargo update -Z direct-minimal-version, to check the dependency version specification declared in Cargo.toml are all valid.

A development shell is also included in the flake, as well as compatibility layers for the traditional default.nix and shell.nix entry points.

[herzogp]

Looks good with CI and more extensive clippy coverage