Allow automatically derived Show for enums to be just the field name

[mitsuhiko]

When working with enums for error reporting it would be super useful if the default implementation of Show for enums could be customized so that you can skip the contained data. For instance if you have a MyError(SomeOtherError) it is interesting to just get MyError instead of MyError(SomeOtherError { foo: 42 }) or whatever is contained in there.

[steveklabnik]

I believe the Display/Debug split takes care of this now, as Display no longer has a default implementation for exactly this reason.