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/bin/tvix-store.rs

@@ -141,7 +141,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                     HashAlgo::Sha256,
                     nar_calculation_service
                         .calculate_nar(&root_node)?
-                        .nar_sha256,
+                        .1
+                        .to_vec(),
                 ));
 
                 let mut drv = Derivation::default();