Add specialized FrozenSet implementations for byte and char

[MihaZupan]

Fixes #110783

Adds 3 new FrozenSet imlpementations:

• ByteFrozenSet - any set of bytes when using the default comparer
• Latin1CharFrozenSet - set of chars <= 255 when using the default comparer
• PerfectHashCharFrozenSet - any set of chars when using the default comparer

I extracted the lookup logic from ProbabilisticMapState in SearchValues to a searate helper so that there's practically no code duplication between the two.

---

Benchmark

public class Bench
{
    private static string Chars() => string.Concat(Enumerable.Range(0, 65536).Select(i => (char)i).Where(c => char.IsAscii(c) && char.IsSymbol(c)));
    private static readonly SearchValues<char> s_sv = SearchValues.Create(Chars());
    private static readonly FrozenSet<char> s_fs = FrozenSet.Create<char>(Chars());
    private static readonly string s_text = File.ReadAllText(@"C:\Users\mihaz\Downloads\gutenberg.org_files_1661_1661-0.txt");

    [Benchmark]
    public int CountSearchValues()
    {
        int count = 0;
        foreach (char c in s_text)
        {
            if (s_sv.Contains(c)) count++;
        }
        return count;
    }

    [Benchmark]
    public int CountFrozenSet()
    {
        int count = 0;
        foreach (char c in s_text)
        {
            if (s_fs.Contains(c)) count++;
        }
        return count;
    }
}

char.IsSymbol (Latin1CharFrozenSet)

Method	Toolchain	Mean	Error	Ratio	Code Size
CountSearchValues		229.9 us	3.19 us		114 B
CountFrozenSet	main	585.6 us	5.93 us	1.00	210 B
CountFrozenSet	pr	228.7 us	2.73 us	0.39	108 B

char.IsAscii && char.IsSymbol (PerfectHashCharFrozenSet)

Method	Toolchain	Mean	Error	Ratio	Code Size
CountSearchValues		265.4 us	3.34 us		102 B
CountFrozenSet	main	1,508.3 us	20.19 us	1.00	141 B
CountFrozenSet	pr	263.6 us	3.53 us	0.17	102 B

char.IsSymbol for bytes (ByteFrozenSet)

Method	Toolchain	Mean	Error	Ratio	Code Size
CountSearchValues		193.5 us	2.60 us		74 B
CountFrozenSet	main	1,529.5 us	22.44 us	1.00	131 B
CountFrozenSet	pr	193.0 us	2.33 us	0.13	74 B

[MihaZupan]

cc: @dotnet/area-system-collections (the bot was lazy)

[stephentoub]

Can you point to some places where FrozenSet<byte> and FrozenSet<char> will be used?

[MihaZupan]

Should be similar to SearchValues<byte>/SearchValues<char>, where you'd previously be using e.g. HashSet / string.Contains / bool[128].
Many uses would benefit from using SearchValues's ContainsAny though as they're just doing scalar loops around Contains themselves.

As a random example, here's one in Uri, though we could of course use SearchValues since we know the implementation:

runtime/src/libraries/System.Private.Uri/src/System/UriHelper.cs

Line 548 in a29d5f9

internal static bool IsNotSafeForUnescape(char ch)

My main motivation was making the answer to "what should I use for Contains(T) checks" simply FrozenSet<T>, given that the interesting part of the implementation can be rather easily shared with SearchValues in this case.