Fix updating GcPtr's internal pointer (get rid of offsetof and use RTTI + pointer to a member?)

[m4drat]

This might require rewriting GcPtr base class to contain pointer to an object?

[m4drat]

https://stackoverflow.com/questions/7464068/base-class-fields-offset

[m4drat]

Even better - move pointer to an object inside GcPtr base class. Make UpdatePtr and GetVoid non-virtual. In the GC loop simply cast to GcPtr and call UpdatePtr on it.