Use numeric sort to compile/test in correct order.
Cleanup 61 thru 69:
- Speed up 61 by checking octagonal values first.
- Use digits rather than strings for 62 and 63.
- Use faster isSquare check for 64.
- Calculate numerator more directly for 65.
- Cleanup 66.
- Overhaul 68 to skip lots of redundant checks.
- Comment 69.
- Implement more figurate sequences in library.