Consider improving FrozenSet for byte/char

[MihaZupan]

FrozenSet<T> is the go-to option if you have an arbitary set of values and just need to do a bunch of Contains(T) queries.
Now that SearchValues<T> is a thing, I've seen the question posed a few times of whether one should use that instead, as it also has a Contains(T).

Currently for the best performance you'd want to use FrozenSet for everything except byte/char, where SearchValues will do a better job.
SearchValues<string>.Contains on the other hand is strictly worse as we're just backing it with a HashSet<string>.

We should consider improving FrozenSet to be as good as SearchValues for these two types to make the decision/recommendations easy.

(part of the issue is also that existing docs on SearchValues don't make it obvious that the point of the type is to use it with extension methods on spans)

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In practice the interesting cases here IMO are:

• using a bitmap when all the values are ASCII
• consider borrowing the perfect-hash O(1) lookup that doesn't need to walk over entries to confirm matches.

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[eiriktsarpalis]

@MihaZupan I put this in the future milestone for now. Feel free to promote to 10 if you plan to work on this yourself.