RFR: 8315850: Improve AbstractMap anonymous Iterator classes [v3]

[Chen Liang]

On Fri, 8 Sep 2023 16:13:16 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR proposes to slightly improve some iterators of `AbstractMap`:
>> 
>> * Declare two fields `final`
>> * Use distinct classes rather than anonymous classes
>
> Per Minborg has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Fix additional formating issue
>  - Don't use polymorphism and reformat code

Marked as reviewed by liach (Author).

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PR Review: https://git.openjdk.org/jdk/pull/15615#pullrequestreview-1632740299