Support nullable as stucture with 2 fields?

[dzmitry-lahoda]

   [TestFixture]
    public class NullableTests
    {
        [Test]
        public void NullableLongByteStruct()
        {
            TypeLayout.PrintLayout<Nullable<LongByteStruct>>();            
            var typeLayout = TypeLayout.GetLayout<Nullable<LongByteStruct>>(includePaddings: true);
            var size = Unsafe.SizeOf<Nullable<LongByteStruct>>();
            Assert.AreEqual(typeLayout.Size, 123);
        }
    }

Standard Output Messages:
 Type layout for 'Nullable`1'
 Size: 24 bytes. Paddings: 0 bytes (%0 of empty space)
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