Delete key operation? about ena

[nikomatsakis]

@dszakallas I don't think we have a way of doing that now. That's kind of a major change to the way the tables work, since the general assumption in the design is that the indices remain valid and are only lazilly updated as needed. In other words, you'd need to do a kind of "global sweep" to ensure that no references remain to those intermediate keys that got created (and of course ena can't know if you have references from outside of its domain). I'm tempted to say that is a feature I would not want to support -- but then I'm also tempted to sort of retire ena or at least try to pull it apart and simplify it.