Improve WebUtility.HtmlEncode / UrlEncode performance

[stephentoub]

• For HtmlEncode, vectorize IndexOfHtmlEncodingChars. Using SearchValues, we can efficiently search for the first ASCII encoding char or the first non-ASCII char, and only then fall back to a scalar loop.
• For HtmlEncode, reduce branching by using a more efficient check to determine whether the ASCII characters need to be encoded.
• For UrlEncode, rather than UTF8-encoding into a new byte[], %-encoding in-place in that, and then creating a string from that, we can use string.Create and just do all the encoding in that buffer.
• For UrlEncode, use SearchValues to vectorize the search for the first non-safe char. Also move the check for ' ' to be inside the if for non-safe char.
• For UrlEncode, use SearchValues to optimize the check for whether an individual character is part of the set (via Contains).
• Simplify IsUrlSafeChar. Rather than multiple checks, one of which is a bitmap, just have a bitmap.
• Remove some leading IsNullOrEmpty checks. Null/empty inputs should be rare, and they're now handled implicitly as part of the subsequent loops.

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Net;

BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);

[MemoryDiagnoser]
public class Tests
{
    [Benchmark]
    [ArgumentsSource(nameof(Inputs))]
    public string HtmlEncode(string input) => WebUtility.HtmlEncode(input);

    [Benchmark]
    [ArgumentsSource(nameof(Inputs))]
    public string UrlEncode(string input) => WebUtility.UrlEncode(input);

    public IEnumerable<object[]> Inputs() =>
    [
        ["this-is-a-very-long-filename-for-an-image-that-should-not-need-encoding.jpg"],
        ["short_name.txt"],
        ["https://www.example.com"],
        ["<test>hello, world</test>"],
        ["לילה טוב"],
        ["""
         How much wood could a woodchuck chuck
         If a woodchuck could chuck wood?
         A woodchuck would chuck as much wood
         As much wood as a woodchuck could chuck,
         If a woodchuck could chuck wood.
         """]
    ];
}

Method	Toolchain	input	Mean	Ratio	Allocated	Alloc Ratio
HtmlEncode	\main\corerun.exe	<test(...)test> [25]	81.576 ns	1.00	96 B	1.00
HtmlEncode	\pr\corerun.exe	<test(...)test> [25]	80.366 ns	0.97	96 B	1.00
UrlEncode	\main\corerun.exe	<test(...)test> [25]	118.515 ns	1.00	160 B	1.00
UrlEncode	\pr\corerun.exe	<test(...)test> [25]	103.039 ns	0.87	96 B	0.60
HtmlEncode	\main\corerun.exe	How (...)ood. [185]	164.359 ns	1.00	-	NA
HtmlEncode	\pr\corerun.exe	How (...)ood. [185]	10.142 ns	0.06	-	NA
UrlEncode	\main\corerun.exe	How (...)ood. [185]	537.945 ns	1.00	664 B	1.00
UrlEncode	\pr\corerun.exe	How (...)ood. [185]	466.832 ns	0.86	432 B	0.65
HtmlEncode	\main\corerun.exe	https(...)e.com [23]	21.442 ns	1.00	-	NA
HtmlEncode	\pr\corerun.exe	https(...)e.com [23]	4.732 ns	0.22	-	NA
UrlEncode	\main\corerun.exe	https(...)e.com [23]	103.053 ns	1.00	136 B	1.00
UrlEncode	\pr\corerun.exe	https(...)e.com [23]	76.013 ns	0.73	80 B	0.59
HtmlEncode	\main\corerun.exe	short_name.txt	13.068 ns	1.00	-	NA
HtmlEncode	\pr\corerun.exe	short_name.txt	4.876 ns	0.38	-	NA
UrlEncode	\main\corerun.exe	short_name.txt	14.324 ns	1.00	-	NA
UrlEncode	\pr\corerun.exe	short_name.txt	3.600 ns	0.26	-	NA
HtmlEncode	\main\corerun.exe	this-(...)g.jpg [75]	62.800 ns	1.00	-	NA
HtmlEncode	\pr\corerun.exe	this-(...)g.jpg [75]	6.975 ns	0.11	-	NA
UrlEncode	\main\corerun.exe	this-(...)g.jpg [75]	70.709 ns	1.00	-	NA
UrlEncode	\pr\corerun.exe	this-(...)g.jpg [75]	5.611 ns	0.08	-	NA
HtmlEncode	\main\corerun.exe	לילה טוב	8.436 ns	1.00	-	NA
HtmlEncode	\pr\corerun.exe	לילה טוב	11.427 ns	1.36	-	NA
UrlEncode	\main\corerun.exe	לילה טוב	100.852 ns	1.00	184 B	1.00
UrlEncode	\pr\corerun.exe	לילה טוב	74.544 ns	0.74	112 B	0.61

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[MihaZupan]

Always love to see SearchValues ratios like these

Method	Toolchain	input	Mean	Ratio	Allocated	Alloc Ratio
HtmlEncode	\main\corerun.exe	How (...)ood. [185]	164.359 ns	1.00	-	NA
HtmlEncode	\pr\corerun.exe	How (...)ood. [185]	10.142 ns	0.06	-	NA