feat(tvix/nix-compat): Add nix serialization support

This change implements the serialization part that is needed to
implement the nix daemon protocol. Previously was add deserialization
and derivers for that and this then adds the other part of that equation
so that you can write types that can then be read using deserialization.

Change-Id: I2917de634980a93822a4f5a8ad38897b9ce16d89
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12729
Autosubmit: Brian Olsen <me@griff.name>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-compat/Cargo.toml

@@ -43,6 +43,7 @@ futures = { workspace = true }
 hex-literal = { workspace = true }
 mimalloc = { workspace = true }
 pretty_assertions = { workspace = true }
+proptest = { workspace = true, features = ["std", "alloc", "tempfile"] }
 rstest = { workspace = true }
 serde_json = { workspace = true }
 smol_str = { workspace = true }