Reduce allocations of ReadString() method

[gmanvel]

This PR reduces allocations of ReadString() method by making use of Span based APIs.

BenchmarkDotNet also shows allocation/perf improvements.

Method	StringLength	Ratio	Alloc Ratio
Existing	256	1.00	1.00
Optimized	256	0.87	0.69
Existing	512	1.00	1.00
Optimized	512	0.87	0.68
Existing	1024	1.00	1.00
Optimized	1024	0.93	0.67
Existing	2048	1.00	1.00
Optimized	2048	0.85	0.67
Existing	4096	1.00	1.00
Optimized	4096	0.82	0.67

Benchmark used

[MemoryDiagnoser(displayGenColumns: false)]
[HideColumns("Mean", "Median", "Error", "StdDev", "RatioSD", "Allocated")]
public class ReaderBenchmarksString
{
    [Params(256, 512, 1024, 2048, 4096)]
    public int StringLength { get; set; }

    private byte[] _bytes;

    [GlobalSetup]
    public void Setup()
    {
        var ms = new MemoryStream();
        var writer = new Writer(ms);
        writer.WriteString(new string('x', StringLength));

        _bytes = ms.ToArray();
    }

    [Benchmark(Baseline = true)]
    public string Existing()
    {
        var reader = new Reader(new MemoryStream(_bytes));
        return reader.ReadString();
    }

    [Benchmark]
    public string Optimized()
    {
        var reader = new Reader(new MemoryStream(_bytes));
        return reader.ReadStringOptimized();
    }
}