Nix packaging for withlang

[siriobalmelli]

Unfortunately efforts to include withlang in nixpkgs are blocked for now: NixOS/nixpkgs#525186

@ehartford would you be amenable to me submitting a patch series introducing nix support?

The general shape would be:

• Toplevel flake.nix and flake.lock files exposing the nix packaging.
• A nix directory containing a package.nix with the "build recipe" (derivation) and any ancillary files needed for the build only.

The advantages to the project this would bring:

• Anyone can try the language immediately with a nix oneliner: nix run github:withlang-dev/with#
• Installing in a nix environment is also a oneliner: nix profile install github:withlang-dev/with#
• Trivial to build and test locally on any Linux/Darwin without any dependency other than nix: nix build
• Trivial to import with into a NixOS configuration (my use case):

  # nix flake
  inputs = {
    # ...
    with.url = "github:withlang-dev/with";
  };

The maintenance burden would be reviewing a PR from me periodically updating dependencies in flake.lock and fixing any build issues that arise.

In future, if I were to be a less attentive maintainer than desired, the reversion path to drop nix support is simply rm -rf flake.* nix.

It goes without saying that any changes proposed outside of the nix surface would be their own commits and not mixed in with nix-specific commits.

What say you?

[ehartford]

Happy to

[siriobalmelli]

PR filed #611; will follow up there