[3.x] Optimize hotspots with Object::derives_from

[lawnjelly]

Un-exposes ancestry (so it becomes internal), and instead exposes Object::_is_class via a rename to Object::derives_from.

Applies in some use cases.

Object::derives_from() should be in a lot of cases 1.5x faster in release builds than using cast_to() (when a NULL check is not necessary), and will be faster in DEV builds (without optimization). It will also be faster than Object::is_class(String) by a huge margin (although we are only worried about hotspots here).

Notes

• This is something discussed with @Ivorforce , we can now totally totally hide the ancestry system as internal, and use it to accelerate Object::cast_to and Object::derives_from.
• These functions have the advantage that they will accelerate the classes in AncestralClass enum / bitfield, but they will also work with all other classes via the backup GDSOFTCLASS system.
• Aim is to do the same in 4.x.
• I haven't converted all the Object::is_class() uses.
  Object::is_class() is still useful for backward compatibility, calling from script (where we don't have an enum of all Object types) and for non-bottleneck areas such as the Editor where we can avoid creating include dependencies.

[Ivorforce]

Should be a straight upgrade.

[lawnjelly]

Thanks!