RFR: 8234502: Merge GenCollectedHeap and SerialHeap

[Lei Zaakjyu]

8234502: Merge GenCollectedHeap and SerialHeap

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Commit messages:
 - merge 'CollectedHeap' and 'SerialHeap'

Changes: https://git.openjdk.org/jdk/pull/16927/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16927&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8234502
  Stats: 2908 lines in 15 files changed: 1431 ins; 1463 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/16927.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16927/head:pull/16927

PR: https://git.openjdk.org/jdk/pull/16927