Fix off-by-one error in `fromXPubAndCount` (#100)
This pull request fixes an off-by-one error in `fromXPubAndCount`. I had skipped this in the initial implementation because generating the initial list of customers is not entirely trivial: The simple expression `fromEnumTo 0 (pred knownCustomerCount)` will not be accepted by the compiler — we have to convince the compiler that `pred` is not applied to `0`.