Introduce StringBuilderPool

[lahma]

To reduce memory allocations and improve scanning speed (less GC) I've introduced StringBuilder pooling. This is basically the same code we've used successfully in Jint and Esprima.

StringBuilderPool returns a disposable read-only struct that tracks lifetime and we access the rented builder from it.

I also sealed OrderedDictionary and made sure it's accessed via concrete type in fields to avoid interface dispatch.

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	97.21 ms	0.592 ms	5833.3333	2833.3333	1000.0000	81 MB
New		83.64 ms (-14%)	0.319 ms	3500.0000 (-40%)	1500.0000 (-47%)	500.0000 (-50%)	53 MB (-35%)

[aaubry]

Very nice. Thanks!

[aaubry]

This feature has been released in version 12.0.0.