Maintain Current Copper Golem Item Placement Behaviour

Currently, when a Copper Golem is placing items in a chest, it prioritises filling the first slot in the chest over completing existing stacks. For example, if there is a stack of 32 iron ingots in the final slot of a chest, and a Copper Golem has 16 iron ingots, and the first slot of the chest is empty, the Copper Golem will put those 16 ingots in the first slot rather than adding them to the existing stack of 32.

This is useful behaviour, and I think it should be kept. This is because a Copper Golem-based sorting system requires 'filter' items to work (these are the items that tell the Copper Golem which chest to sort which items in to). It is important that these 'filter' items remain separate from the items being sorted. Otherwise, there is a risk that the 'filter' items are mixed up with other items and removed from the sorting system, breaking it. 

The current behaviour allows the items being sorted to be removed from a chest with a hopper without also removing the 'filter' items. This is because hoppers prioritise the first slot of a chest, so you can put the 'filter' items in the final slots of a chest to protect them from being removed by a hopper, whilst Copper Golems will put the sorted items in the first slots of the chest where they can be removed. This is especially useful when you want to filter multiple different types of items into the same chest, as comparators (which previous sorting systems used to keep filter items) only test for item amount, not item type.