IntrusivePtr follow-up

[JeanPhilippeKernel]

This follow-up PR provides some fixes and improvements of existing features.

• fixed the use of noexcept for method and operators
• added support of intrusive ptr creation between base and derived classes
• switch from std::is_convertible to std::convertible_to to support implicit and explicit conversion
• merged into one header file of IntrusivePtr and IntrusiveWeakPtr
• extended RefCounted class to support Strong and Weak ref