RFR: 8254883: ZGC: Make the ZArrayIterator reusable for ZRelocationSetIterators

[Per Liden]

The ZRelocationSetIterators are mostly just duplicate implementations of the ZArrayIterators. With some slight
adjustments to ZArrayIteratorImpl we can make that code reusable for ZRelocationSetIteratorImpl.

Note that this PR builds/depends on "8254878: Move last piece of ZArray to GrowableArray", which is out for review as a
separate PR (https://github.com/openjdk/jdk/pull/694).

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Commit messages:
 - 8254883: ZGC: Make the ZArrayIterator reusable for ZRelocationSetIterators
 - 8254878: Move last piece of ZArray to GrowableArray

Changes: https://git.openjdk.java.net/jdk/pull/697/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=697&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254883
  Stats: 138 lines in 7 files changed: 38 ins; 70 del; 30 mod
  Patch: https://git.openjdk.java.net/jdk/pull/697.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/697/head:pull/697

PR: https://git.openjdk.java.net/jdk/pull/697