fix(tvix/castore): B3Digest::{to_vec -> as_slice}

Not a single call site actually makes use of the Vec.

Change-Id: I6cf31073c9f443d1702a21937a0c3938c2c643b8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9988
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/castore/src/digests.rs

@@ -15,9 +15,8 @@ pub enum Error {
 pub const B3_LEN: usize = 32;
 
 impl B3Digest {
-    // returns a copy of the inner [Vec<u8>].
-    pub fn to_vec(&self) -> Vec<u8> {
-        self.0.to_vec()
+    pub fn as_slice(&self) -> &[u8] {
+        &self.0[..]
     }
 }