Merge Fitness traits with Individual

[dbalcomb]

This merges the Fitness and FitnessMut traits with Individual.

The original intent of having a separate Fitness trait is that some types, such as vectors and arrays, have no obvious fitness value without adding new restrictive trait bounds to infer it from the items. The FitnessMut trait was intended to allow the fitness to be mutated without requiring all individuals allow fitness mutation. However, after #94 the API was changed to return a reference and so by ruling out computed fitness values there is less need for FitnessMut. The only concern there would be allowing the fitness of a primitive to be mutated as that would alter the individual itself and by extension the genome.

This change merges the Fitness and FitnessMut traits into the Individual trait and renames the associated Value to Fitness. This simplifies the API at the cost of allowing the scoring of a primitive such as an integer to alter the individual itself and not just the score.

This includes a new Nil fitness value for individuals that do not have an actual fitness. This makes use of an empty array to construct a static reference to a value that works with both shared and exclusive references. This could be improved with unsafe or perhaps an external crate to construct a ZST but at the moment it only works for empty arrays and slices.