x values of the solutions to the three equations are now generated using additions instead of multiplications.

- Created an iterator, `MultiplesOfSquares`, to generate the multiples of square numbers.
 - Added in a new file, `square_multiples.rs`.
 - Added unit tests in `multiples_test.rs`.
- Replaced all `x` loops in the atkin sieve with loops over the new iterator.
 - This allowed some rearranging of the loops.
  - The loops are now grouped by multiples of squares of `x`, rather than by `x` values which are then multiplied in the loops.