Simplify sequence generation w/ iterate and scanl.
Rewrite 84.
- Fix a presumed logic bug where chance #10 says go back 3 spots,
  but not necessarily to do what is there (i.e. CH3 -> CC3).
- Use Markov chains instead of simulation (slower, but more accurate).
- Use doubles instead of ratio, they have enough precision and
  are faster with the increased computation.