It would be very helpful to have an efficient implementation of
enumeration set constants, to replace existing uses of bit masks.

Collections.unmodifiableSet called on an EnumSet
currently creates a set that is two layers of wrapping around a long.
Operations using this immutable enum set can be quite inefficient.
For example, given

   public static final Set<Letter> vowels =
Collections.unmodifiableSet(EnumSet.of(A, E, I, O, U));

and

   Set<Letter> x = EnumSet.noneOf(Letter.class);

the implementation of

   x.addAll(vowels);

reverts to the very inefficient iterator version rather than a
bit-based implementation.

This improvement could be done without adding any API elements, by
specializing the implementation in Collections.unmodifiableSet.