RFR: 8276093: Improve naming in closures to iterate over card sets

[Thomas Schatzl]

Hi all,

  can I have reviews for this renaming change that tries to change naming for some remembered set closures to include `Closure` instead of `Iterator` in their names. Additionally I removed two or so `G1` prefixes for nested classes. Hopefully this improves the situation a bit, although it's still a mess of nested closures.

Testing: local compilation, gha

Thanks,
  Thomas

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Commit messages:
 - first attempt

Changes: https://git.openjdk.java.net/jdk/pull/6430/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6430&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276093
  Stats: 30 lines in 4 files changed: 1 ins; 0 del; 29 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6430.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6430/head:pull/6430

PR: https://git.openjdk.java.net/jdk/pull/6430