Update digit library functions.
Prefer digit extraction over 'show'.
Sort imports.
Cleanup 40 thru 45.
- 116 is broken / stuck, add temporary workaround.
- Add triangular, pentagonal, and hexagonal generators.
- 40 and 41 now use digits instead of strings.
- 43 uses a generic function instead of a giant list comprehension.
- Explain why 44 is technically cheating.
- 45 walks lists in parallel.