core library updates for enhanced enums

[devoncarew]

See #47849 for the main tracking issue.

[lrhn]

The platform library Enum declaration needs to change in small ways. Those changes should not be a problem to add early, since users can't reach them anyway.

The implementations may also want to have an _Enum class which is updated as well. We'll have to lock down the implementation design first.

[itsjustkevin]

@lrhn was there an associated issue where this work was done? Could we please update this issue?

[lrhn]

I don't think any changes were necessary. Later iterations of the design made it an error to override index, so there was no need to change the existing implementation in order to move that to a superclass.

So, done.