fn work(&mut self, fuel: &mut isize) {
        // We only perform work for merges in progress.
        if let MergeState::Double(layer) = self {
            layer.work(fuel)
        }
    }
Performs a bounded amount of work towards a merge.

If the merge completes, the resulting batch is returned.
If a batch is returned, it is the obligation of the caller
to correctly install the result.
differential-dataflow/src/trace/implementations/spine_fueled.rs