Add parameter list ref usage analyzer for low level reader/writer

[Wraith2]

While trying to write a custom formatter for a complex type I hit a problem where the write phase was producing illegal output which seemed to be caused by a missing field write. I have now tracked this down to a missing ref on an extension method.

        public static void Write(this /* ref */ MessagePackWriter writer, int? value)
        {
            if (value.HasValue)
            {
                writer.Write(value.Value);
            }
            else
            {
                writer.WriteNil();
            }
        }

The MessagePackWriter is a struct for performance reasons and if the ref is omitted a copy will happen here. The write advance is applied to the copy and then lost when the method returns. This leads to the write vanishing in this case. This is a subtle and hard to diagnose pit of failure.

I suggest that an analyzer be added looking at the use of MessagePackReader and MessagePackWriter in parameter lists which check to see if they are being passed by ref and if they aren't producing an error.

[AArnott]

Not a bad idea. If you send a PR for it yourself, be sure you target the develop branch, as there have been a lot of changes between that and master (our default branch).