Add toString() to kotlin.Enum

Overridable because it is in java.lang.Enum
Make enum classes comparable

 #KT-3727 Fixed
Prohibit all modifiers for enum entries

 #KT-2679 Fixed
Regenerate test data on enums

Now they have final equals, hashCode and compareTo
Minor, fix some warnings in DeclarationsChecker
Make 'when' on nullable enums exhaustive if 'null' entry is present

 #KT-2902 Fixed
Fix incorrect "senseless null in when" on nullable type
Prohibit type parameters for enum classes

 #KT-5696 Fixed