Add convenience methods for List/Set/Map of values into Associations

In the UnitOfWork, we should probably add the following methods,

<T> void toAssociations( ManyAssociation<T> assoc, List<T> values );
<T> void toAssociations( ManyAssociation<T> assoc, Set<T> values );
<T> void toAssociations( NamedAssociation<T> assoc, Map<T> values );

For each element in 'values', call the toEntity() method in UnitOfWork, and then add it to the Association. The toEntity() will do all the heavy lifting of creating/updating the underlying entity store.