This PR exposes an array field as a ReadOnlySpan.

This was motivated by the need to store a GUID in a field. The natural way to do this is using a field of 16 bytes, which means the way to extract and encode it requires copying to an intermediate array. The C# GUID struct provides a ReadOnlySpan constructor, allowing us to create a GUID without copying to an intermediate byte buffer, e.g.:

var guid = new Guid(message.Guid);

and for encoding store:

var guidBytes = Guid.NewGuid().ToByteArray();

then set it using:

message.Guid = new ReadOnlySpan<byte>(guidBytes);

I've also added an AsSpan() method, to support writing directly from a Guid object to the underlying array. This allows you to write a Guid like this:

guid.TryWriteBytes(message.GuidAsSpan());