Swapper

I added the new Structure Swapper. Similar to the Splitter it connects to two input and two output Belts.
But instead of trying to distribute Items in a fair manner, it just swaps the Items of the Belts.

            +---------+
Belt --->---|         |===>=== Belt
            |         |
            | Swapper |
            |         |
Belt ===>===|         |--->--- Belt
            +---------+