message: Make CallStack::pop into a debug_assert
  Summary: Only the Starlark library can modify the call stack, so check it in debug, but don't bother in release.
diff: @@ -100,7 +100,7 @@ impl<'v> CallStack<'v> {
  /// Remove the top element from the stack. Called after `push`.
  pub(crate) fn pop(&mut self) {
  let old = self.stack.pop();
  - assert!(
  + debug_assert!(
  old.is_some(),
  "CallStack.pop() called without preceding push()"
  )
  