Optimize most likely sequence (2x speedup)

Sped things up a bit. Problem 1 when I started was talking 10,000ms, by the time I finished I got it down to 4,700ms. Not too shabby!

The two largest things were:
  1. Removing function calls for trans_prob
  2. Caching this [dict lookup](https://github.com/ianks/hidden-markov-model/compare/optimize?expand=1#diff-b343eabc493922845adff1edbe80dbb4R50)