feat(tvix/nix-compat): add a basic listing deserializer

.ls files are useful to seek in a NAR without parsing it entirely.

The responsibility of validating the files is on the caller.

Change-Id: I5d1da28b5479c38f20ca5babe60e362a2217c9ea
Signed-off-by: Ryan Lahfa <tvl@lahfa.xyz>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12196
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/nix-compat/src/nar/mod.rs

@@ -1,4 +1,5 @@
 pub(crate) mod wire;
 
+pub mod listing;
 pub mod reader;
 pub mod writer;