Fix memory management issue in static property

This pull request is to fix defect [MAGN-336 array modification inside function crashes due to garbage collection in the following case] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-336). 

It is because when assigning a value to a static property, the reference count of that value is not increased. ``TypeSystem.Coerce()`` may allocate a new array if it needs to do type coercion for elements in an array, so ``GCRetain()`` should be called on the return value from ``TypeSystem.Coerce()``. 

Notice register ``FX`` is never used, so some code clean up for ``FX`` register. 

Regression test case Regression_MAGN336 is added. 

Run language test cases to ensure no regression. 
Yes, I dont see any places that the value in ``FX`` register is used.