RFR: 8315850: Improve AbstractMap anonymous Iterator classes [v4]

[Per Minborg]

> 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 one additional commit since the last revision:

  Remove comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15615/files
  - new: https://git.openjdk.org/jdk/pull/15615/files/1057e166..ceafcaf5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15615&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15615&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15615.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15615/head:pull/15615

PR: https://git.openjdk.org/jdk/pull/15615