feat(tvix/eval): Support builtins.attrNames

Define `.len()` method on `NixAttrs` to preallocate the capacity of the result
vector.

Also anchor an errant comment to its context (I think).

Change-Id: I268f15025d453d7b3ae1146558c80e51433dd2a8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6546
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -51,6 +51,18 @@ fn pure_builtins() -> Vec<Builtin> {
                 args.pop().unwrap().to_str()?.as_str().to_owned(),
             ));
         }),
+        Builtin::new("attrNames", 1, |args, vm| {
+            force!(vm, &args[0], value, {
+                let xs = value.to_attrs()?;
+                let mut output = Vec::with_capacity(xs.len());
+
+                for (key, _val) in xs.iter() {
+                    output.push(Value::String(key.clone()));
+                }
+
+                Ok(Value::List(NixList::construct(output.len(), output)))
+            })
+        }),
         Builtin::new("catAttrs", 2, |mut args, _| {
             let list = args.pop().unwrap().to_list()?;
             let key = args.pop().unwrap().to_str()?;