Testing some singleton implementations

[Stanislav Blinov]

There's a lot more to these singletons than meets the eye.

- It would seem that such usage of raw MemoryOrder in 
AtomicSingleton would be wrong (e.g. return to acq/rel is in 
order, which should not pose any performance issues on X86, as 
Sean mentioned).

- The instance references should be qualified shared.

This needs more serious review, even if only for academic 
purposes. I'll see what I can come up with :)
In the meantime, if anyone has anything to add to the list, 
please chime in!