NAR and NAR export serialization code

[adnelson]

Maybe related to #128, maybe not, but I thought I'd ask:

I have code for serializing and deserializing NARs and "exported" NARs (including metadata) that I reverse engineered from the C++ implementation a while ago. It currently lives here: https://github.com/adnelson/nix-binary-cache/blob/master/src/Nix/Nar/Serialization.hs
I'm not sure if this already being worked on, or where it would fit into the hnix code if we wanted it here, but I'd be happy to merge them into this one.

[mightybyte]

Pinging @imalsogreg

[adnelson]

Maybe this would actually be better suited to the hnix-store repo... 🤔

[adnelson]

Posted there, closing here :)