feat(tvix/eval): add Value variants for strings & attrsets

Change-Id: Idebf663ab7fde3955aae50f635320f7eb6c353e8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6087
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/value/attrs.rs

@@ -34,3 +34,9 @@ impl Display for NixAttrs {
         f.write_str("}")
     }
 }
+
+impl PartialEq for NixAttrs {
+    fn eq(&self, _other: &Self) -> bool {
+        todo!("attrset equality")
+    }
+}