[RogueC] Check for instanceOf in singleton assignment
[RogueC] Check for instanceOf in singleton assignment

- Previously any object could be assigned to a singleton even if it was
  not instanceOf the singleton type.

Plasmaworks/Rogue,Plasmaworks/Rogue,AbePralle/Rogue,Plasmaworks/Rogue,Plasmaworks/Rogue,AbePralle/Rogue