Statistics tracking over time

[deontologician]

Mostly, I don't actually care what the all-time total of statistics are. What I really want is to track statistics over time.

Stats to keep as a rate:

• children born / second
• kills / second
• matings / second: should correlate with children born, but they can diverge a lot.
• rounds / second

Stats to not keep anymore

• mutations: this is a direct function of the mutation rate and the birth rate and isn't too interesting. Plus it requires we bubble up stats from deep in the mutation code.
• feeders_eaten / second: this just isn't a very interesting stat

The idea is that we have the saver thread from #47 that we'll dump these stats to, and it can write them to disk in some format that allows us to save every data point (probably should be efficient, append-only and be cap-able in size.