refactor(tvix/store/directorysvc): use [u8; 32] instead of Vec<u8>

Also, simplify the trait interface, only allowing lookups of Directory
objects by their digest.

Change-Id: I6eec28a8cb0557bed9b69df8b8ff99a5e0f8fe35
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8313
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/import.rs

@@ -85,7 +85,7 @@ fn process_entry<BS: BlobService, CS: ChunkService + std::marker::Sync, DS: Dire
                 .to_str()
                 .map(|s| Ok(s.to_owned()))
                 .unwrap_or(Err(Error::InvalidEncoding(entry.path().to_path_buf())))?,
-            digest: directory_digest,
+            digest: directory_digest.to_vec(),
             size: directory_size,
         }));
     }