Instead of references. This is needed because those function pointers
are used in GDNative which needs to work with plain C, which doesn't
support passing parameters by reference.
- Initialize Object pointer to nullptr so it's not used by mistake.
- When setting an Object check if it's a reference so refcounting works
as intended.
-Moved Expression to use this, removed its own.
-Eventually GDScript/VisualScript/GDNative need to be moved to this.
-Given the JSON functions were hacked-in, removed them and created a new JSONParser class
-Made sure these functions appear properly in documentation, since they will be removed from GDScript
The underscore prefix was used to avoid the conflict between the `RID` class
name and the matching enum value in `Variant::Type`.
This can be fixed differently by prefixing uses of the `RID` class in `Variant`
with the scope resolution operator, as done already for `AABB`.
