Use Int for capacity in TBQueue interface again?

[konsumlamm]

In #17 / 2221948, the type for the capacity of TBQueues was changed from Int to Natural, to avoid negative capacities. With #70, the capacity is now internally stored as an Int (since array lengths are Ints), so IMO it would make sense to change the public API back to Ints. Moreover, we need to check that the capacity is greater than 0 anyway and currently we additionally have to check that the capacity is not greater than maxBound :: Word. This would also avoid potentially having to unbox/box Naturals.