Add internal interfaces for the different types of enum

[Crell]

interface Enum
interface UnitEnum extends Enum  { public function cases() }
interface ScalarEnum extends Enum { public function value(), public static function from() }
interface TaggedEnum extends Enum

[Crell]

RFC has been updated accordingly. @iluuu1994, mark this closed when it's been implemented.

[iluuu1994]

According to Nikita my parser workaround for allowing Enum is not sufficient. I think we should drop the Enum interface for now. It is redundant as we now have is_enum.

[Crell]

I think I already removed the Enum interface from the RFC itself.

[iluuu1994]

It's still under "New interfaces"

[Crell]

Removed.

[iluuu1994]

Thanks 🙂