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/lib.rs

@@ -1,3 +1,4 @@
+mod digests;
 mod errors;
 
 pub mod blobservice;
@@ -7,6 +8,7 @@ pub mod nar;
 pub mod pathinfoservice;
 pub mod proto;
 
+pub use digests::B3Digest;
 pub use errors::Error;
 
 #[cfg(test)]