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/tests/nar_renderer.rs

@@ -198,7 +198,7 @@ fn test_complicated() {
             &mut buf,
             crate::proto::node::Node::Directory(DirectoryNode {
                 name: "doesntmatter".to_string(),
-                digest: DIRECTORY_COMPLICATED.digest(),
+                digest: DIRECTORY_COMPLICATED.digest().to_vec(),
                 size: DIRECTORY_COMPLICATED.size(),
             }),
         )