Release v0.4.0

[fizruk]

This version introduces sections and variables. The feature is similar to Variable command in Coq. An important difference, however, is that rzk does not allow definitions to use variables implicitly and adds uses (...) annotations to ensure such dependencies are not accidental.

• Variables and sections (Coq-style) (see #38);

Minor improvements:

• Add flake, set up nix and cabal builds, cache nix store on CI (see #39);
• Apply stylish-haskell (see 7d42ef62);