Remove Comparable from ItemStack, add Comparators for specific orderings

There's no simple ordering that is consistent with the likely behavior of ItemStack.equals(), and the most useful orderings can be easily defined via the ItemStack interface.
I didn't really have very good names for the comparators. They're supposed to be type + damage + size and type + size. Type alone may also be a useful one, as might type + damage but then the names start to get a bit too long.