Override Read(Span)/ReadByte on PipeReaderStream

[stephentoub]

We can do better than the base implementations for reading. For writing, there's not a lot we can do better than the base.

Fixes #53692
cc: @halter73

Method	Toolchain	Mean	Ratio	Allocated
ReadByte	\main\corerun.exe	542,053.1 ns	1.00	131,072 B
ReadByte	\pr\corerun.exe	423,396.2 ns	0.78	0 B
ReadSpan	\main\corerun.exe	459.7 ns	1.00	72 B
ReadSpan	\pr\corerun.exe	309.6 ns	0.67	0 B

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Columns;
using BenchmarkDotNet.Configs;
using BenchmarkDotNet.Diagnosers;
using BenchmarkDotNet.Reports;
using BenchmarkDotNet.Running;
using Perfolizer.Horology;
using System;
using System.Globalization;
using System.IO;
using System.IO.Pipelines;

[MemoryDiagnoser]
public class Program
{
    public static void Main(string[] args) =>
        BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args,
            DefaultConfig.Instance.WithSummaryStyle(new SummaryStyle(CultureInfo.InvariantCulture,
                printUnitsInHeader: false, sizeUnit: SizeUnit.B, timeUnit: TimeUnit.Nanosecond, printZeroValuesInContent: true)));

    const int Length = 4096;

    private Stream _writer, _reader;
    private byte[] _buffer = new byte[Length];

    public Program()
    {
        var pipe = new Pipe();
        _writer = pipe.Writer.AsStream();
        _reader = pipe.Reader.AsStream();
    }

    [Benchmark]
    public void ReadByte()
    {
        _writer.WriteAsync(_buffer).GetAwaiter().GetResult();
        for (int i = 0; i < Length; i++)
        {
            _reader.ReadByte();
        }
    }

    [Benchmark]
    public void ReadSpan()
    {
        _writer.WriteAsync(_buffer).GetAwaiter().GetResult();
        Span<byte> buffer = _buffer;
        while (!buffer.IsEmpty)
        {
            buffer = buffer.Slice(_reader.Read(buffer));
        }
    }
}

[adamsitnik]

LGTM, that is a very nice improvement!