refactor(tvix/nix-compat): make NixHash an enum with fixed-len bytes

Less Vec<u8> passed around.

Change-Id: Ie153a6bfaa084d7490ffa38634efdf5f3c31a768
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9722
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-compat/src/derivation/tests/mod.rs

@@ -238,17 +238,16 @@ fn output_path_construction() {
         Output {
             path: "".to_string(), // will be calculated
             hash_with_mode: Some(crate::nixhash::NixHashWithMode::Recursive(
-                (
+                crate::nixhash::from_algo_and_digest(
                     crate::nixhash::HashAlgo::Sha256,
-                    data_encoding::HEXLOWER
+                    &data_encoding::HEXLOWER
                         .decode(
                             "08813cbee9903c62be4c5027726a418a300da4500b2d369d3af9286f4815ceba"
                                 .as_bytes(),
                         )
                         .unwrap(),
                 )
-                    .try_into()
-                    .unwrap(),
+                .unwrap(),
             )),
         },
     );