Add 'for (_, foo) in bar.enumerate()' rule

[ceeK]

This would be useful as I see this in code reviews every now and again.

People no longer need to use the index returned from enumerate, and xcode tells them to ignore "_" the variable.

Instead, enumerate should be removed.

Not sure if I should file a radar for this to be added directly to xcode, so opening it here first for discussion.

[jpsim]

I could see a rule like this added. Although it's not quite as general as this ticket implies, since that's only relevant for certain SequenceTypes, but certainly the most common ones like Array and Set.