Testing some singleton implementations

[Stanislav Blinov]

In fact #2, I think it's even safe to pull that store out of the 
synchronized block:

         // (2)
         if (!atomicLoad!(MemoryOrder.raw)(_instantiated))
         {
             // (1)
             synchronized
             { // <- this is 'acquire'
                 if (_instance is null) {
                     _instance = new AtomicSingleton;
                 }

             } // <- this is 'release'

             // This store cannot be moved to positions (1) or (2) 
because
             // of 'synchronized' above
             atomicStore!(MemoryOrder.raw)(_instantiated, true);
         }