RFR: 8342214: GenShen: Reduce code duplication in shFreeSet with iterator abstraction

[William Kemper]

Introduce `ShenandoahLeftRightIterator` and `ShenandoahRightLeftIterator` classes to encapsulate allocation bias to one side of the heap or the other.

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Commit messages:
 - Use iterator abstraction to reduce code duplication

Changes: https://git.openjdk.org/shenandoah/pull/512/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=512&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342214
  Stats: 195 lines in 2 files changed: 80 ins; 69 del; 46 mod
  Patch: https://git.openjdk.org/shenandoah/pull/512.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/512/head:pull/512

PR: https://git.openjdk.org/shenandoah/pull/512