Cleverness to merge extra-conf etc

[Hoverbear]

We should safely handle when a user adds options which we also set.

[Hoverbear]

So Nix accepts this format: https://metacpan.org/pod/Config::General

Which I am not sure if there is a parser in Rust...

[Hoverbear]

I almost wonder if it's worth trying to teach Nix to be configured with another format as well...

[cole-h]

I was able to hack up a library that roughly parses the Nix config as Nix itself does: https://github.com/DeterminateSystems/nix-config-parser (Nix's implementation here: https://github.com/NixOS/nix/blob/0079d2943702a7a7fbdd88c0f9a5ad677c334aa8/src/libutil/config.cc#L80-L138)

For the moment, it's little more than a re-implementation of the parser code from Nix that puts the settings into a HashMap<String, String>. Maybe we could make that more useful (maybe make NixConfig a proper struct with helper methods for various things), if that would help.