feat(tvix/castore/digests): impl From digest::Output<_> for B3Digest

This allows calling .into() to get a B3Digest.

Change-Id: I6e63b496413cd00d84acfcd15c7de0f64c79721f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11086
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/castore/src/digests.rs

@@ -26,6 +26,13 @@ impl From<B3Digest> for bytes::Bytes {
     }
 }
 
+impl From<digest::Output<blake3::Hasher>> for B3Digest {
+    fn from(value: digest::Output<blake3::Hasher>) -> Self {
+        let v = Into::<[u8; B3_LEN]>::into(value);
+        Self(Bytes::copy_from_slice(&v))
+    }
+}
+
 impl TryFrom<Vec<u8>> for B3Digest {
     type Error = Error;