feat(tvix/nix-compat/store_path): derive Hash for StorePathRef

This allows StorePathRef as keys in a hashmap, by deriving Hash. The same is already done for StorePath.

Change-Id: I3fc54c45787948116dcb27dfb5dc806b9b505835
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11269
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

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

@@ -183,7 +183,7 @@ impl Serialize for StorePath {
 /// Like [StorePath], but without a heap allocation for the name.
 /// Used by [StorePath] for parsing.
 ///
-#[derive(Debug, Eq, PartialEq, Clone, Copy)]
+#[derive(Debug, Eq, PartialEq, Clone, Copy, Hash)]
 pub struct StorePathRef<'a> {
     digest: [u8; DIGEST_SIZE],
     name: &'a str,