feat(tvix/eval): Support builtins.length

Get the length of a list

Change-Id: I41d91e96d833269541a1b3c23b7cc879f96d1e5a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6407
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/list.rs

@@ -28,6 +28,10 @@ impl NixList {
         lhs
     }
 
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
     pub fn construct(count: usize, stack_slice: Vec<Value>) -> Self {
         debug_assert!(
             count == stack_slice.len(),