feat(nix-compat/{aterm,derivation}): init parser

This provides a nom-based parser for Nix derivations in ATerm format,
which can be reached via `Derivation::from_aterm_bytes`.

Some of the lower-level ATerm primitives are moved into a (new) aterm
module, and some more higher-level ones that construct derivation-
specific types.

Also, move the escape_bytes function into there, this is a generic ATerm
thing.

Change-Id: I2b03b8a1461c7ea2fcb8640c2fc3d1fa3ea719fb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9730
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/nix-compat/src/nixhash/mod.rs

@@ -81,7 +81,7 @@ pub fn from_algo_and_digest(algo: HashAlgo, digest: &[u8]) -> Result<NixHash> {
 }
 
 /// Errors related to NixHash construction.
-#[derive(Debug, thiserror::Error)]
+#[derive(Debug, Eq, PartialEq, thiserror::Error)]
 pub enum Error {
     #[error("invalid hash algo: {0}")]
     InvalidAlgo(String),