feat(tvix/store/proto): use Bytes instead of Vec<u8>

Makes use of https://github.com/tokio-rs/prost/pull/341, which makes our
bytes field cheaper to clone.

It's a bit annoying to configure due to
https://github.com/hyperium/tonic/issues/908, but the workaround does
get the job done.

Change-Id: I25714600b041bb5432d3adf5859b151e72b12778
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8975
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>

tvix/eval/src/builtins/impure.rs

@@ -38,8 +38,10 @@ mod impure_builtins {
         let dir = generators::request_read_dir(&co, path).await;
         let res = dir.into_iter().map(|(name, ftype)| {
             (
-                // TODO: propagate Vec<u8> into NixString.
-                NixString::from(String::from_utf8(name).expect("parsing file name as string")),
+                // TODO: propagate Vec<u8> or bytes::Bytes into NixString.
+                NixString::from(
+                    String::from_utf8(name.to_vec()).expect("parsing file name as string"),
+                ),
                 Value::String(
                     match ftype {
                         FileType::Directory => "directory",