RFR: Add generations to freeset [v8]

[Kelvin Nilsen]

On Thu, 20 Apr 2023 20:02:52 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> If you are doing the `expand_` and `adjust_` at each insertion or removal from sets, I'd encourage you to think about encapsulating intervals as a performance heuristic into the set abstraction, and always keep them updated, or have an API call that adjusts them if you are batching the interval adjustments to amortize the cost of these adjustments over several insertions and deletions.
>
> I think I'll go ahead and make suggested changes and take a shot at a poor man's set abstraction.

Latest PR reduces replication of code.  It does not go all the way to a new ShenandoahFreeRegionSet abstraction.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/250#discussion_r1174062024