Add ItemStack array serialization methods

[kennytv]

Serializing multiple items to bytes isn't trivial, so often beginners fall into the trap of using output IO streams and Bukkit serialization instead, so these provide helpful extra methods to deal with item arrays.

Also kept the serialization methods exposed in API as a sort of textbook example/nudge in the right direction of what should be used instead of object streams

cyclic these imports

[lynxplay]

Brought up in a merge party in VC, but iirc the general notion was to move away from null item stacks in the API as they require a double check of item != null && !item.isEmpty().

Would it make more sense for this method to yield back ItemStack#empty() instead of null if 0 is read?

[kennytv]

While the api output still provides null items, I would not enforce air, otherwise you either produce inconsistent results or break easy use from api input