[Bleeding] Handle custom furnace recipes seperately. Fixes BUKKIT-1657, BUKKIT-3846

Due to vanilla blanket comparing data values, and the unspecified order of
hashmap iterators, we need to run through custom recipes first, and
therefore seperately, to ensure that they are actually used.  By not
adding the custom results to the experience table, we do not override the
experience gains from vanilla smelting recipes.