refactor(nix-compat/nar/reader): reuse prev_name allocation

We reuse the prev_name allocation for Entry, instead of allocating and
returning a separate Vec.

We encode the `prev_name: None` case as an empty vector, since we don't
allow empty names anyway, and the sorting is equivalent.

Change-Id: I975b37ff873805f5ff099bc82128706891052247
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11607
Reviewed-by: Brian Olsen <me@griff.name>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/store/src/nar/import.rs

@@ -87,7 +87,7 @@ where
                 let mut path = path.clone();
 
                 // valid NAR names are valid castore names
-                path.try_push(&entry.name)
+                path.try_push(entry.name)
                     .expect("Tvix bug: failed to join name");
 
                 let entry = Box::pin(produce_nar_inner(