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/mod.rs

@@ -17,6 +17,8 @@ pub use grpc_blobservice_wrapper::GRPCBlobServiceWrapper;
 pub use grpc_directoryservice_wrapper::GRPCDirectoryServiceWrapper;
 pub use grpc_pathinfoservice_wrapper::GRPCPathInfoServiceWrapper;
 
+use crate::B3Digest;
+
 tonic::include_proto!("tvix.store.v1");
 
 #[cfg(feature = "reflection")]
@@ -238,10 +240,15 @@ impl Directory {
 
     /// Calculates the digest of a Directory, which is the blake3 hash of a
     /// Directory protobuf message, serialized in protobuf canonical form.
-    pub fn digest(&self) -> [u8; 32] {
+    pub fn digest(&self) -> B3Digest {
         let mut hasher = blake3::Hasher::new();
 
-        *hasher.update(&self.encode_to_vec()).finalize().as_bytes()
+        let vec = hasher
+            .update(&self.encode_to_vec())
+            .finalize()
+            .as_bytes()
+            .to_vec();
+        B3Digest::from_vec(vec).unwrap()
     }
 
     /// validate checks the directory for invalid data, such as: