RFR: 8258810: Improve enum traits

[Albert Mingkun Yang]

On Tue, 22 Dec 2020 18:45:31 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review these miscellaneous improvements to the enum iteration facility.
> Thanks to @albertnetymk for discussions.
> 
> (1) Removed some unneeded stuff from EnumIterationTraits.
> 
> (2) Various similar bits of error checking code are unified and shared via
> EnumIterationTraits.
> 
> (3) Casts between the enum type and the underlying type are now done using
> helper functions in EnumIterationTraits.
> 
> (4) JDK-8257466 added accessors for the EnumRange bounds and the index for
> an enumerator, but didn't account for the possibility of an empty subrange.
> Add assertions for that case.
> 
> (5) Added some death tests to verify some of the error checking.
> 
> Testing:
> mach5 tier1

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.

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Marked as reviewed by ayang (Author).

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