Allow models without a mutex

[sourcefrog]

Currently, Nutmeg always wraps the model in a mutex.

Some models don't need this, e.g. if they use atomics or otherwise do their own locking. It would be nice to have an option to not use the mutex.

Perhaps these models should have to be Send+Sync, but then conceivably Nutmeg could be used from just a single thread, in which case we wouldn't need that. So, perhaps we should just let those traits propagate to the view.