If a circular relationship is set up between two models using
 `setRelation()` (or similar methods) then calling
 `$model->relationsToArray()` will call `toArray()` on each related
 model, which will in turn call `relationsToArray()`. In an instance
 where one of the related models is an object that has already had
 `toArray()` called further up the stack, it will infinitely recurse
  down and result in a stack overflow.

The same issue exists with `getQueueableRelations()`, `push()`, and
 potentially other methods. This adds tests which will fail if one of
 the known potentially problematic methods gets into a recursive loop.

This adds a trait for Eloquent which can be used to prevent recursively
 serializing circular references.

…fault