RFR: 8199318: add idempotent copy operation for Map.Entry [v2]

[Daniel Fuchs]

On Wed, 2 Jun 2021 17:54:06 GMT, Stuart Marks <smarks at openjdk.org> wrote:

>> I'm fixing this along with a couple intertwined issues.
>> 
>> 1. Add Map.Entry::copyOf as an idempotent copy operation.
>> 
>> 2. Clarify that AbstractMap.SimpleImmutableEntry is itself unmodifiable (not really immutable) but that subclasses can be modifiable.
>> 
>> 3. Clarify some confusing, historical wording about Map.Entry instances being obtainable only via iteration of a Map's entry-set view. This was never really true, since anyone could implement the Map.Entry interface, and it certainly was not true since JDK 1.6 when SimpleEntry and SimpleImmutableEntry were added.
>
> Stuart Marks has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Tiny editorial tweaks.

Marked as reviewed by dfuchs (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/4295