ARC: fix ARC errors in TestCrossPersistentRootReferences, and add a FIXME about an error in COLibrary. This one looks kind of serious; generally, the pattern of returning a COObject from a factory method which creates a transient COObjectGraphContext inside that factory method will not work with ARC, since ARC is free to deallocate the COObjectGraphContext at the end of the method body (turning the COObject into a zombie immediately). It's a pain but the best thing I have found is to make such factory methods return a COObjectGraphContext, or have a COObjectGraphContext as an ivar in the factory object (probably better). 

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10250 7b3f36aa-e6db-49c1-8b4c-66eac1997e64