put
Adds a T instance to the cache and attaches a toggle reference.
- If the object is floating, it is sunk ( - refSink()).
- A toggle reference is added and the object is unreferenced. 
- A Cleaner ensures proper removal when the object is GC'ed. 
Return
The existing or newly added instance.
Parameters
T
The type of the instance being cached.
proxy
The instance to be cached.