Generalize range hierarchy with a trait

[bvssvni]

The basic idea is that Vec<T> points to Vec<U> where T is a range. Could use https://github.com/pistondevelopers/range

To make it type safe, the Range struct can be wrapped in a type that implements a trait Parent with an associated type set to U. Generic algorithms can then be written to operate on the relationship between parent and children. This could be added to piston-range.