Consider adding EntityManager#link/unlink methods for direct association manipulation

A problem when working with collection-valued associations is that almost all operations except add($obj) require the collection to become initialized in order for the operation to be performed properly. While this is all correct and beautiful OO-wise it may be problematic at times with regards to performance. Hence we might want to consider to provide some convenient methods along the lines of link/unlink (name suggestions?) which allow more direct, less OO collection manipulation. Such methods obviously would bypass the normal object lifecycle and the changes done through these methods will not be reflected in the in-memory objects and collections, unless the user keeps them in-synch himself.