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/glue/src/builtins/fetchers.rs

@@ -61,7 +61,7 @@ impl FetchArgs {
         name: Option<String>,
         sha256: Option<String>,
         mode: HashMode,
-    ) -> nixhash::Result<Self> {
+    ) -> nixhash::NixHashResult<Self> {
         Ok(Self {
             name: name.unwrap_or_else(|| url_basename(&url).to_owned()),
             url,