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/nix-compat/Cargo.toml

@@ -10,6 +10,7 @@ anyhow = "1.0.68"
 data-encoding = "2.3.3"
 glob = "0.3.0"
 serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
 sha2 = "0.10.6"
 thiserror = "1.0.38"