eliminate all but one way of replacing Rule context

[mscarey]

currently there's Procedure.new_context() and the entire concept of a Holding object.

Probably I need to stick with Procedure.new_context(). The Holding object at first glance seems to link all Holdings with the same meaning to the same ProceduralRule object, but realistically, there's no way to prevent multiple ProceduralRule objects with the same meaning from being created, so the Holdings class just adds a confusing abstraction that doesn't solve the deduping problem or any other problem.

[mscarey]

Eliminated Holding class 03638a0