Fix for 12540: Use Object.init.sizeof to avoid recursive alias definitio...

...ns

Fixes https://d.puremagic.com/issues/show_bug.cgi?id=12540

I had to put the unittest in a version(unittest) block because forward referencing `Class12540` does not work inside of a `unittest {}` block (and there's no code to run here; the 'test' is just to ensure that this case compiles).