refactor(tvix/nix-compat): absorb nar writer

Expose it at `nix_compat::nar::writer`.

Change-Id: I98a8acfa6277074f24608d64fb6c0082b41d10f5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7969
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/nix-compat/src/lib.rs

@@ -1,3 +1,4 @@
 pub mod derivation;
+pub mod nar;
 pub mod nixbase32;
 pub mod store_path;