feat(tvix/eval): add NixList::force_elements()

This adds a function NixList::force_elements() which forces each
element of the list shallowly.  This behavior is needed for
`builtins.replaceStrings`, and probably a few other builtins as
well.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: I3f0681acbbfe50e781b5f07b6a441647f5e6f8da
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7094
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/value/list.rs

@@ -100,6 +100,11 @@ impl NixList {
 
         Ok(true)
     }
+
+    /// force each element of the list (shallowly), making it safe to call .get().value()
+    pub fn force_elements(&self, vm: &mut VM) -> Result<(), ErrorKind> {
+        self.iter().try_for_each(|v| v.force(vm).map(|_| ()))
+    }
 }
 
 impl IntoIterator for NixList {