Pool or otherwise remove some buffers in StringAsPinnedUTF8 on NET6+

[stevenaw]

This PR targets .NET6+ but all of the targeted APIs are also available on lower runtimes if dependencies were to be taken on NuGet packages such as System.Buffers.

The PR avoids double-allocation of buffers and adds array pooling to the needed buffer to reduce memory usage to a consistent value independent of payload size while also improving a bit of the performance.

Benchmarks (using .NET8 runtime)

| Method   | N   | Mean     | Error     | StdDev   | Ratio | RatioSD | Gen0   | Allocated | Alloc Ratio |
|--------- |---- |---------:|----------:|---------:|------:|--------:|-------:|----------:|------------:|
| Original | 11  | 36.63 ns | 15.401 ns | 0.844 ns |  1.00 |    0.00 | 0.0166 |     104 B |        1.00 |
| Updated  | 11  | 34.86 ns |  1.437 ns | 0.079 ns |  0.95 |    0.02 | 0.0051 |      32 B |        0.31 |
|          |     |          |           |          |       |         |        |           |             |
| Original | 64  | 39.95 ns |  2.437 ns | 0.134 ns |  1.00 |    0.00 | 0.0331 |     208 B |        1.00 |
| Updated  | 64  | 41.82 ns | 14.281 ns | 0.783 ns |  1.05 |    0.02 | 0.0051 |      32 B |        0.15 |
|          |     |          |           |          |       |         |        |           |             |
| Original | 128 | 46.58 ns | 15.269 ns | 0.837 ns |  1.00 |    0.00 | 0.0535 |     336 B |        1.00 |
| Updated  | 128 | 42.75 ns | 10.538 ns | 0.578 ns |  0.92 |    0.02 | 0.0051 |      32 B |        0.10 |

        [MemoryDiagnoser]
    public class Utf8Pinner
    {
        // 11 = "Hello World" :)
        [Params(11, 64, 128)]
        public int N { get; set; }

        public string Payload { get; set; }

        [GlobalSetup]
        public void Setup()
        {
            Payload = new string('A', N);
        }


        [Benchmark(Baseline = true)]
        public IntPtr Original()
        {
            using var pinner = new OriginalUTF8Pinner(Payload);
            return pinner.Ptr;
        }

        [Benchmark]
        public IntPtr Updated()
        {
            using var pinner = new UpdatedUTF8Pinner(Payload);
            return pinner.Ptr;
        }

        public sealed class UpdatedUTF8Pinner : IDisposable
        {
            private readonly GCHandle gch;

#if NET6_0_OR_GREATER
            private readonly byte[] strBytesNulTerminated;

            public UpdatedUTF8Pinner(string str)
            {
                var size = Encoding.UTF8.GetMaxByteCount(str.Length);

                // Ask for one extra for the null byte
                strBytesNulTerminated = ArrayPool<byte>.Shared.Rent(size + 1);

                try
                {
                    Span<byte> slice = strBytesNulTerminated.AsSpan().Slice(0, size);
                    int bytesWritten = Encoding.UTF8.GetBytes(str, slice);

                    // 0-init the remainder
                    Array.Clear(strBytesNulTerminated, bytesWritten, strBytesNulTerminated.Length - bytesWritten);
                }
                catch
                {
                    // An exception above means the object never creates, potentially leaving a
                    // memory leak as there would be no object to Dispose() of later
                    ArrayPool<byte>.Shared.Return(strBytesNulTerminated);
                    throw;
                }

                this.gch = GCHandle.Alloc(strBytesNulTerminated, GCHandleType.Pinned);
            }

#else
            public UpdatedUTF8Pinner(string str)
            {
                byte[] strBytes = System.Text.UTF8Encoding.UTF8.GetBytes(str);
                byte[] strBytesNulTerminated = new byte[strBytes.Length + 1]; // initialized to all 0's.
                Array.Copy(strBytes, strBytesNulTerminated, strBytes.Length);
                this.gch = GCHandle.Alloc(strBytesNulTerminated, GCHandleType.Pinned);
            } 
#endif

            public IntPtr Ptr { get => this.gch.AddrOfPinnedObject(); }

            public void Dispose()
            {
                gch.Free();
#if NET6_0_OR_GREATER
                ArrayPool<byte>.Shared.Return(strBytesNulTerminated);
#endif
            }
        }

        public sealed class OriginalUTF8Pinner : IDisposable
        {
            private GCHandle gch;

            public OriginalUTF8Pinner(string str)
            {
                byte[] strBytes = System.Text.UTF8Encoding.UTF8.GetBytes(str);
                byte[] strBytesNulTerminated = new byte[strBytes.Length + 1]; // initialized to all 0's.
                Array.Copy(strBytes, strBytesNulTerminated, strBytes.Length);
                this.gch = GCHandle.Alloc(strBytesNulTerminated, GCHandleType.Pinned);
            }

            public IntPtr Ptr { get => this.gch.AddrOfPinnedObject(); }

            public void Dispose()
            {
                gch.Free();
            }
        }
    }

</details>