RFR: Add generations to freeset [v23]

[Kelvin Nilsen]

> ShenandoahFreeSet has not yet been modified to deal efficiently with the combination of old-gen and young-gen collection set reserves.  This PR makes changes so that we can distinguish between collector_is_free, old_collector_is_free, and mutator_is_free.  Further, it endeavors to keep each set of free regions tightly packed, so the range of regions representing each set is small.
> 
> In its current form, this no longer fails existing regression tests (except for known problems that are being addressed independently)

Kelvin Nilsen has updated the pull request incrementally with two additional commits since the last revision:

 - Reviewer feedback: fix capitalization in comment
 - Reviewer feedback: add a TODO comment

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/250/files
  - new: https://git.openjdk.org/shenandoah/pull/250/files/d967f503..b112fdd9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=250&range=22
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=250&range=21-22

  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/250.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/250/head:pull/250

PR: https://git.openjdk.org/shenandoah/pull/250