feat(tvix/eval): introduce initial Lambda type

Change-Id: Ifa9766f5ffeff99e926936bafd697e885e733b78
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6238
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/value/lambda.rs

@@ -0,0 +1,12 @@
+//! This module implements the runtime representation of functions.
+use std::rc::Rc;
+
+use crate::chunk::Chunk;
+
+use super::NixString;
+
+#[derive(Clone, Debug)]
+pub struct Lambda {
+    name: Option<NixString>,
+    chunk: Rc<Chunk>,
+}