day 22: Lower bound mana, prune based on bound
day 22: Lower bound mana, prune based on bound

We can calculate how much mana it would take to kill the boss using
poisons and magic missiles and use that to determine whether we have any
chance of beating the current best.

This is a very slight speedup, maybe an improvement from 0.2 to 0.18
seconds.

It decreases the number of nodes searched as well:
part1 9950 -> 6276
part2 7081 -> 5150

petertseng/adventofcode