feat(tvix/eval): implement list concatenation

Change-Id: Icdf715d116371a9f139bdf95266410bf967bef25
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6144
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -81,6 +81,16 @@ impl Value {
             }),
         }
     }
+
+    pub fn as_list(self) -> EvalResult<NixList> {
+        match self {
+            Value::List(l) => Ok(l),
+            other => Err(Error::TypeError {
+                expected: "list",
+                actual: other.type_of(),
+            }),
+        }
+    }
 }
 
 impl Display for Value {