It looks like the only tension is between `Stopwatch#lap` and `Stopwatch#nextLap`. I'm thinking of two solutions:

1. Update `Stopwatch#lap` to also return the internal representation of a lap. This seems sub-optimal, because we're exposing internal information just for consistency's sake--I don't think the view ever needs to know a lap's `time`.
2. Update `Stopwatch#nextLap` to return just the duration, and move the `time` calculation to `Stopwatch#lap`.

What do you think?