RFR: 8258810: Improve enum traits [v2]

[Kim Barrett]

On Thu, 24 Dec 2020 20:44:37 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   ayang review
>
> I think `assert(size() > 0)` is more consistent with preceding documentation than `assert_not_empty()` in `first()` and `last()`. Very subjective though; up to you.

Thanks @albertnetymk and @iklam for reviews.  Still need another reviewer, since @albertnetymk is not yet a committer.

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PR: https://git.openjdk.java.net/jdk/pull/1868