feat(tvix/eval): implement ? operator (single-level only)

This makes it possible to check things like `{} ? a` with a single
level of nesting.

Change-Id: I567c36fcfd2f9e2f60071acd3ebfe56dea59b26f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6161
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/vm.rs

@@ -173,6 +173,13 @@ impl VM {
                     }
                 }
 
+                OpCode::OpAttrsIsSet => {
+                    let key = self.pop().as_string()?;
+                    let attrs = self.pop().as_attrs()?;
+                    let result = Value::Bool(attrs.select(key.as_str()).is_some());
+                    self.push(result);
+                }
+
                 OpCode::OpList(count) => {
                     let list =
                         NixList::construct(count, self.stack.split_off(self.stack.len() - count));