feat(tvix/eval): implement NixList::iter

This does not require a custom iterator type (for now?)

Change-Id: I5beb194bd8629571bd4040c69c977c27149807fa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6371
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/value/list.rs

@@ -39,6 +39,10 @@ impl NixList {
         NixList(stack_slice)
     }
 
+    pub fn iter(&self) -> std::slice::Iter<Value> {
+        self.0.iter()
+    }
+
     pub fn into_iter(self) -> std::vec::IntoIter<Value> {
         self.0.into_iter()
     }