Taxicab numbers

Implement Taxicab numbers.
It would be nice to write a separate function containing the logic of the program. This way it becomes easier to read and to test (so we can get rid of `// not_tested`).

I think the best solution would be an infinite iterator through all taxicab numbers. This way you could use `take` to get the first 25, then `skip` and `take` again to get those from 2000 to 2006.