feat(tvix/eval): add Value::as_attrs method

Change-Id: I2f39122ac85b67837335aab308d845907160e132
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6221
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/value/mod.rs

@@ -62,6 +62,16 @@ impl Value {
         }
     }
 
+    pub fn as_attrs(&self) -> EvalResult<&NixAttrs> {
+        match self {
+            Value::Attrs(attrs) => Ok(attrs),
+            other => Err(Error::TypeError {
+                expected: "set",
+                actual: other.type_of(),
+            }),
+        }
+    }
+
     pub fn to_string(self) -> EvalResult<NixString> {
         match self {
             Value::String(s) => Ok(s),