day 22: Don't key cache by turn
day 22: Don't key cache by turn

It matters not what turn we see a certain state! We only care about the
cost when we reach that state.

Running in -b mode gives the same output, so this gives extra assurance
that this is correct.

This has improved -b mode runtime from ~51 seconds to ~46 seconds.
This was based on one run of each so not the most scientific, but I'm
not really in a mood to run this multiple times right now.

A separate run printing out the number of seen prunes indicates that the
number of prunes hasn't changed.

A separate run printing out whenever the cache has seen the same state
across multiple turns also prints out nothing.

The above two points indicate that splitting the cache by turn never
made any difference anyway. So this is just removing a level of
indirection from the cache, which should be a slight speedup.

petertseng/adventofcode