Add alternative to values for enums that return immutable indexed sequence without defensive copying

[bjornregnell]

The method values on enums returns an Array that gets allocated at each call. This is inline with how Java enums work.

It would be nice if an alternative method existed, for example called seq, that returned an ArraySeq that is just a shared immutable reference to the already allocated sequence of values, not requiring a defensive copying. This would be more efficient and also more Scala-idomatic.

[bjornregnell]

See also gitter conversation here: https://gitter.im/scala/contributors?at=60f5bcd9049a0c3e5380f64d