Add Fitness trait

[dbalcomb]

This adds a new Fitness trait that is different to the previous Fitness trait.

The Fitness trait was originally introduced in #29 as an extension to the Individual trait to provide a new fitness method. The trait was named after the method but didn't quite fit the naming conventions in this crate or the wider Rust ecosystem. At the time it seemed preferable to WithFitness, Fit, Score, or any other option. Now that the Fitness trait has been merged with the Individual trait in #97 the name is free to be repurposed.

This change introduces a new Fitness trait that represents the value instead of an individual with a fitness. The trait has a single method nil which was chosen based on the Nil fitness from #98 and to avoid conflicting with the zero method from the Zero trait. Unfortunately types such as Reverse do not implement Zero and it required Add so that was not an option. The Default trait was ruled out because that may imply a non-zero value. However, nil may suggest no value rather than a zero value so there is no clear option. At the very least the method justifies the existence of the trait.

This also updates various trait bounds to replace Default with Fitness and updates the associated Fitness in the Individual trait to add a new bound.