refactor(tvix/eval): introduce Closure struct in Value type

This struct will carry the upvalue machinery in addition to the lambda
itself. For now, all lambdas are wrapped in closures (though
technically analysis of the environment can later remove innermost
Closure wrapper, but this optimisation may not be worth it).

Change-Id: If2b68549ec1ea4ab838fdc47a2181c694ac937f2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6269
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/value/function.rs

@@ -0,0 +1,28 @@
+//! This module implements the runtime representation of functions.
+use std::rc::Rc;
+
+use crate::chunk::Chunk;
+
+#[derive(Clone, Debug)]
+pub struct Lambda {
+    // name: Option<NixString>,
+    pub(crate) chunk: Rc<Chunk>,
+}
+
+impl Lambda {
+    pub fn new_anonymous() -> Self {
+        Lambda {
+            // name: None,
+            chunk: Default::default(),
+        }
+    }
+
+    pub fn chunk(&mut self) -> &mut Rc<Chunk> {
+        &mut self.chunk
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct Closure {
+    pub lambda: Lambda,
+}