feat(tvix/value): add runtime representation of simple lists

There might be more logic in the future to encapsulate different
backing implementations of lists as well.

Change-Id: Ib7064fab48bf88b0c8913b0ecfa2108177c7c9fd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6093
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>

tvix/eval/src/value/list.rs

@@ -0,0 +1,14 @@
+/// This module implements Nix lists.
+use std::fmt::Display;
+
+use super::Value;
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct NixList(pub Vec<Value>);
+
+impl Display for NixList {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        // TODO(tazjin): format lists properly
+        f.write_fmt(format_args!("<list({})>", self.0.len()))
+    }
+}