refactor(tvix/nix-compat): -derivation::Hash, +NixHash

This stops using our own custom Hash structure, which was mostly only
used because we had to parse the JSON representation somehow.

Since cl/8217, there's a `NixHash` struct, which is better suited to
hold this data. Converting the format requires a bit of serde labor
though, but that only really matters when interacting with JSON
representations (which we mostly don't).

Change-Id: Idc5ee511e36e6726c71f66face8300a441b0bf4c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8304
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/Cargo.nix

@@ -3610,6 +3610,10 @@ rec {
             packageId = "serde";
             features = [ "derive" ];
           }
+          {
+            name = "serde_json";
+            packageId = "serde_json";
+          }
           {
             name = "sha2";
             packageId = "sha2 0.10.6";