add basic shell.nix

[redianthus]

Hi @mlemerre,

It allows me to run dune build @all -p codex and to successfully run the tests.

I plan to package both cudd and pacomb in nixpkgs in the future (and probably codex too).

I did not bother with binsec_codex and frama_c_codex for now.

• pacomb PR on nixpkgs: ocamlPackages.pacomb: init at 1.4.3 NixOS/nixpkgs#494796
• cudd PR on nixpkgs: ocamlPackages.cudd: init at 0.1.3 NixOS/nixpkgs#494805

[mlemerre]

Hi Leo,

We already have a shell.nix file in devenv/nix/shell.nix (and nix packages for pacomb and stuff). They are meant for our CI so they may not be optimal (and I don't know nix very well), and maybe yours is better suited? WDYT?

[redianthus]

Yes, I think they could probably be made simpler but I did not wanted to disturb your CI process.

I think it's fine to have both for now. I can try to make them converge towards a unique file in the future if you agree. Having this simple one first makes it easier for me to contribute. (it's better if the shell.nix is at the root of the repo, it's a convention used a lot and it allows me to simply cd codex and have the right environment setup automatically)

EDIT: pacomb and cudd are now part of nixpkgs, I simplified the file a little bit to use the packaged version.