No way to combine item stacks in the inventory

[DaveyBiggers]

We can swap stacks around, but there's no way to combine them at the moment - eg if there are twelve blocks of granite in slot 0, and ten blocks of granite in slot 1, we should be able to combine them to get a stack of 22 granite blocks.

We need a command along the lines of
"combineInventoryItems 13 14"
to combine the stacks 13 and 14, if possible, and leave the result in slot 13.