Declare an array proxy property for ordered relationships

This change defines a new `readonly` property of the form `(name)Array` for each ordered to-many relationship which returns the ordered set's `-array` proxy. It also defines ordered insertion and removal methods for `(name)Array` sufficient to make `-mutableArrayValueForKey:` work. This combination seems to allow you to bind an ordered relationship to an `NSArrayController` (though I wouldn't recommend mutating the relationship through one of the other accessors).