Various cleanups.
- Use new primeFactors in 3, 47, and 72.
- Drop allDivisors and allDivisorsP from library.
- Use divisorPowerSum 0 in 12.
- Use divisorPowerSum 1 in 21 and 23.
- Overhaul 108 and 110 to use divisorPowerSum 0 and use identical code.
- Rebuild Euler.o when make flags change.