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/nix-compat/src/wire/bytes/mod.rs

@@ -82,7 +82,6 @@ where
     Ok(buf)
 }
 
-#[allow(dead_code)]
 pub(crate) async fn read_bytes_buf<'a, const N: usize, R: ?Sized>(
     reader: &mut R,
     buf: &'a mut [MaybeUninit<u8>; N],