Crate reference_counted_singleton

RefCountedSingleton is a reference-counted singleton whose protected data can be recreated as needed.

The protected data is created when RefCountedSingleton::get_or_init is called. That functions returns an RCSRef reference to the singleton.

RCSRef instances can be cloned as needed. The last RCSRef reference drops the data. Calling RefCountedSingleton::get_or_init again recreates the data.

Structs

RCSRef	Read-only counted reference to an instance of RefCountedSingleton.
RefCountedSingleton	A reference-counted singleton whose protected data can be recreated as needed.