Reference counting with fat pointers

[Kagamin via Digitalmars-d]

Looks like I got the core logic working.

The scheme takes advantage of D type system and distinction 
between shared and unshared data to decide on interlocked 
reference counting at compile time. Unshared mutable data and 
shallow shared data (like strings) are counted with the fastest 
possible non-atomic arithmetic. Cycles should be handled by the 
user, see an example at the end. Language interface is not 
considered, assess if the approach itself is ok.