Cleanup 72 thru 74:
- Push memoized digitFactorial into library, use it in 34.
- Use totient sieve to greatly speed 72.
- Speed up 73 by neither building fractions nor range checking.
- Speed up 74 by storing next chain items in an array.
More cleanup / speedup:
- Fix bug in 61.
- Use tree in 74.
- Sort and group list for 75, array benefits are negligible.
- Implement partition search in library, use it for 76 and 78.
- Big speedup for 76, small slowdown for 78.
- Cleanup temporary link files on 'make clean'