Add yield to pollardfactor

[DannyKong12]

As per #167 adding yields allows factor and pollardfactor a scheduler to run when factor is wrapped in an @task. Adding a yield here means the execution can be monitored from time to time by the scheduling code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.00%. Comparing base (42b97a5) to head (ffe33de).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
- Coverage   94.20%   94.00%   -0.21%     
==========================================
  Files           2        2              
  Lines         466      467       +1     
==========================================
  Hits          439      439              
- Misses         27       28       +1     

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