feat(tvix/nix-compat): implement Serialize, Deserialize for NixHash

We use the (slightly more tolerant) from_str to deserialize, and
serialize out as SRI.

Change-Id: If76b0ed2d4e243904f02df34f6c90b976c0bab8c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11393
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/nix-compat/src/derivation/parser.rs

@@ -48,7 +48,7 @@ pub(crate) fn parse(i: &[u8]) -> Result<Derivation, Error<&[u8]>> {
 fn from_algo_and_mode_and_digest<B: AsRef<[u8]>>(
     algo_and_mode: &str,
     digest: B,
-) -> crate::nixhash::Result<CAHash> {
+) -> crate::nixhash::NixHashResult<CAHash> {
     Ok(match algo_and_mode.strip_prefix("r:") {
         Some(algo) => nixhash::CAHash::Nar(nixhash::from_algo_and_digest(
             algo.try_into()?,