refactor(tvix/store): use bytes for node names and symlink targets

Some paths might use names that are not valid UTF-8. We should be able
to represent them.

We don't actually need to touch the PathInfo structures, as they need to
represent StorePaths, which come with their own harder restrictions,
which can't encode non-UTF8 data.

While this doesn't change any of the wire format of the gRPC messages,
it does however change the interface of tvix_eval::EvalIO - its
read_dir() method does now return a list of Vec<u8>, rather than
SmolStr. Maybe this should be OsString instead?

Change-Id: I821016d9a58ec441ee081b0b9f01c9240723af0b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8974
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/impure.rs

@@ -38,7 +38,8 @@ mod impure_builtins {
         let dir = generators::request_read_dir(&co, path).await;
         let res = dir.into_iter().map(|(name, ftype)| {
             (
-                NixString::from(name.as_str()),
+                // TODO: propagate Vec<u8> into NixString.
+                NixString::from(String::from_utf8(name).expect("parsing file name as string")),
                 Value::String(
                     match ftype {
                         FileType::Directory => "directory",