This will make it possible fairly easily use Nix to represent
arbitrary data structures, e.g. for using Nix as a config language.
Only pure Nix (i.e. no `import` etc.) is supported for now.
Not all types, specifically no struct traversal, are implemented in
this commit.
Change-Id: I9ac91a229a0d12bf818e6e3249f3e5a691599a2c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7712
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
