Generate extra toMany accessors like CoreData does

[helje5]

The CoreData generator also generates such accessors for toMany properties:

extension GroupOfItems {

    @objc(insertObject:inItemsAtIndex:)
    @NSManaged public func insertIntoItems(_ value: Item, at idx: Int)

    @objc(removeObjectFromItemsAtIndex:)
    @NSManaged public func removeFromItems(at idx: Int)

    @objc(insertItems:atIndexes:)
    @NSManaged public func insertIntoItems(_ values: [Item], at indexes: NSIndexSet)

    @objc(removeItemsAtIndexes:)
    @NSManaged public func removeFromItems(at indexes: NSIndexSet)

    @objc(replaceObjectInItemsAtIndex:withObject:)
    @NSManaged public func replaceItems(at idx: Int, with value: Item)

    @objc(replaceItemsAtIndexes:withItems:)
    @NSManaged public func replaceItems(at indexes: NSIndexSet, with values: [Item])

    @objc(addItemsObject:)
    @NSManaged public func addToItems(_ value: Item)

    @objc(removeItemsObject:)
    @NSManaged public func removeFromItems(_ value: Item)

    @objc(addItems:)
    @NSManaged public func addToItems(_ values: NSOrderedSet)

    @objc(removeItems:)
    @NSManaged public func removeFromItems(_ values: NSOrderedSet)

Note that those are @NSManaged!

Should be easy?