Devise a mechanism for "deprecating" Op names #271

We need a mechanism for Op developers to deprecate Op names as we move Ops towards better standards while maintaining backwards compatibility. Ideally, these Ops would continue to be matched using deprecated names, but should not be searchable under those names.

To enable this, we require:

• A mechanism for denoting that a particular Op name is deprecated (should it be distributed, written on every Op, or centralized, a property of the OpEnvironment?)
• Alterations within the OpEnvironment to prevent the proliferation of those names (is it enough to just alter DefaultOpDescriptionGenerator? Probably, for now...)