refactor(tvix/store/directorysvc): move from Vec<u8> to B3Digest

This introduces a new struct, B3Digest, which internally holds a
Vec<u8>, but only allows construction with 32 bytes.

It also implements display, which will print the base64 representation.
This should reduce some boilerplate when parsing Vec<u8>.

Change-Id: Ia91aa40cb691916773abc8f93e6ed79a5fd34863
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8592
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/store/src/proto/grpc_pathinfoservice_wrapper.rs

@@ -70,7 +70,10 @@ impl<
         match request.into_inner().node {
             None => Err(Status::invalid_argument("no root node sent")),
             Some(root_node) => match self.nar_calculation_service.calculate_nar(&root_node) {
-                Ok(resp) => Ok(Response::new(resp)),
+                Ok((nar_size, nar_sha256)) => Ok(Response::new(proto::CalculateNarResponse {
+                    nar_size,
+                    nar_sha256: nar_sha256.to_vec(),
+                })),
                 Err(e) => Err(e.into()),
             },
         }