New Cop: Avoid usage of `Class.descendents`

[bensheldon]

There are two reasons why this is an unreliable solution:

• it doesn't know about things that have yet to be autoloaded
• it's non-deterministic with regards to Garbage Collection of classes. If you use Class.descendants in Test, where there is a pattern to dynamically define classes, GC is unpredictable for when those classes are cleaned up and removed by the GC.

cc @jhawthorn