RFR: Add generations to freeset [v8]

[Kelvin Nilsen]

On Thu, 20 Apr 2023 18:15:15 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> Once again, the consolidation/ecapsulation of all this into the set abstraction would reduce the triplication of the code, and thrice repeated lines that must be kept in sync for any change, increasing maintenance burden.
>
> 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.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/250#discussion_r1173035705