Parse optional visibility for enum item

Syntactically enum items can have a visibility. The visibility has to
be removed (through a cfg attribute or macro) before they get lowered.
The semantic checking will be done when we implement lowering enum items.

Make the AST EnumItem class a VisItem. This simplifies things a little
for cloning items, handling outer attributes and will help when adding
EnumItem (sub)classes to AST visitors (so they can be handled as
Items). Also add a get_identifier method to Enum and EnumItem.

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