Usability of ARC/ORC in multi threaded code.

[PMunch]

Hmm, this "solution" considerably lowers the usability of the isolation system 
though.. Even the original issue just calls for better alias detection. If we 
had better alias detection the first of my two usecases would be possible 
today. With the rules changes to an `and` I think the isolation system is 
dramatically reduced in usefulness simply because of how many data structures 
in Nim uses references.