RFR: 8338477: GenShen: Cleanup generational heap [v3]

[William Kemper]

> * Inline heap accessor
> * break up huge do_work method
> * remove TODOs

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Improve comment for relativizing stack chunks of evacuated objects

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/474/files
  - new: https://git.openjdk.org/shenandoah/pull/474/files/5c6261c6..9116643d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=474&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=474&range=01-02

  Stats: 10 lines in 1 file changed: 9 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/474.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/474/head:pull/474

PR: https://git.openjdk.org/shenandoah/pull/474