notnft - write nftables rules in nix

Honestly I don't know why I made this, it clearly isn't worth it lol (Jesus, 5000kloc for this), but it was interesting to learn more about nftables (I also found a bug in Nix in the process). I will contribute to nftables docs some time later, as I found many undocumented or poorly documented parts, and some documentation that's flat out wrong.

Currently this can only export rules in json, but it shouldn't be too hard to write a transpiler into the nftables language.