[Safe code] Clean up Guid.TryFormatX

[EgorBo]

Contributes to #94941

This PR removes unsafe code from Guid.TryFormatX + small clean up in TryParseExact.

New version is 10-15% faster.

[EgorBo]

@EgorBot -amd -arm -profiler

using System.Runtime.CompilerServices;
using BenchmarkDotNet.Attributes;

public class Benchmark
{
    static Guid guid = Guid.Parse("{D420C46B-04DC-4ABD-BF32-5132E82EE7F7}");

    [Benchmark]
    public bool FormatX()
    {
        Span<char> buffer = stackalloc char[100];
        bool result = guid.TryFormat(buffer, out int charsWritten, "X");
        Consume(buffer.Slice(0, charsWritten));
        return result;
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static void Consume(Span<char> _){}
}

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