Finish implementation of augmented assignment.
Finish implementation of augmented assignment.

Add support for list += list and list *= int, which modifies the lvalue directly.

Add support for __setattr__ hook.

Factor out common code.

LivingLogic/LivingLogic.Javascript.ul4,doerwalter/LivingLogic.Javascript.ul4,LivingLogic/LivingLogic.Javascript.ul4,doerwalter/LivingLogic.Javascript.ul4