RFR: DRAFT: Add generations to freeset [v3]

[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.
> 
> As currently drafted, there are regressions represented by assertion failures and crashes.  The purpose of publishing this draft PR is to allow careful scrutiny of the code.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Fix calculation of minimum fill size
  
  We were incorrectly using byte size rather than word size.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/250/files
  - new: https://git.openjdk.org/shenandoah/pull/250/files/d991cd7c..837dc51b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=250&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=250&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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