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

This will save us some copies, because a clone will simply create an
additional pointer to the same data.

Change-Id: I017a5d6b4c85a861b5541ebad2858ad4fbf8e8fa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8978
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/store/src/proto/mod.rs

@@ -247,12 +247,11 @@ impl Directory {
     pub fn digest(&self) -> B3Digest {
         let mut hasher = blake3::Hasher::new();
 
-        let vec = hasher
+        hasher
             .update(&self.encode_to_vec())
             .finalize()
             .as_bytes()
-            .to_vec();
-        B3Digest::from_vec(vec).unwrap()
+            .into()
     }
 
     /// validate checks the directory for invalid data, such as: