Change Mark to be readonly struct

[lahma]

!! This is binary breaking change !!

To reduce memory allocations and improve scanning speed (less GC) I've changed Mark to be a read-only struct. In parsing test code profiling showed that over 20% of memory allocations were from constructing Mark instances. This is a binary breaking change, but I'd argue the performance benefits are quite good. The API itself didn't change

Using in modifier passing Mark instances around as struct size exceeds pointer size.

Also introduced ThrowHelper to allow inlining Mark constructor.

Benchmark Code

Uses the large YAML from issue #519 . I noticed you don't have a benchmark project currently so I'm running my local one.

[MemoryDiagnoser]
public class YamlStreamBenchmark
{
    private string yamlString = "";

    [GlobalSetup]
    public void Setup()
    {
        yamlString = File.ReadAllText(@"c:\work\saltern.yml");
    }

    [Benchmark]
    public void Load()
    {
        var yamlStream = new YamlStream();
        yamlStream.Load(new StringReader(yamlString));
    }
}

Results

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.22000
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK=6.0.100
  [Host]     : .NET 6.0.0 (6.0.21.52210), X64 RyuJIT
  DefaultJob : .NET 6.0.0 (6.0.21.52210), X64 RyuJIT

YamlStreamBenchmark

Diff	Method	Mean	Error	Gen 0	Gen 1	Gen 2	Allocated
Old	Load	96.71 ms	1.551 ms	5833.3333	2833.3333	1000.0000	81 MB
New		73.13 ms (-24%)	1.319 ms	4428.5714 (-24%)	2000.0000 (-29%)	714.2857 (-29%)	63 MB (-22%)

[aaubry]

I noticed you don't have a benchmark project currently so I'm running my local one.

I used to have one to compare the performance across releases, but it was difficult to maintain and provided little value so it was removed. Feel free to add a benchmark if you feel it would be useful.

[lahma]

@aaubry would you like it as part of this PR or separate?

[aaubry]

@aaubry would you like it as part of this PR or separate?

As you prefer. I was about to merge this, but I can wait.

[lahma]

Please merge, I'll create a separate one! 😄

[aaubry]

This feature has been released in version 12.0.0.