Use stackalloc to reduce allocations on NewUlid

[andrewjsaid]

Benchmark:

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using CSharp.Ulid;

BenchmarkSwitcher
    .FromAssembly(typeof(Program).Assembly)
    .Run(new[] { "-i" });

[MemoryDiagnoser]
public class Benchmarks
{
    [Benchmark]
    public Ulid NewUlid() => Ulid.NewUlid();
}

Before:

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3296/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.100
  [Host] : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

Toolchain=InProcessEmitToolchain  

Method	Mean	Error	StdDev	Gen0	Allocated
NewUlid	50.55 ns	0.526 ns	0.492 ns	0.0024	40 B

After:

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3296/23H2/2023Update/SunValley3)
AMD Ryzen 9 5950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 8.0.100
  [Host] : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

Toolchain=InProcessEmitToolchain  

Method	Mean	Error	StdDev	Allocated
NewUlid	43.51 ns	0.259 ns	0.243 ns	-