Currently SimpleMemoryUsageTracker and MemoryUsageTracker don't have a handy way to reset the amount of tracked memory to 0. It seems really useful to have this.

Also, should we update these classes so that the amount of memory being tracked goes to zero when the destructor fires? This seems to overlap a bit with the intention of the Token classes.

As we begin to instrument stages with memory tracking, we should consider what features these classes could use to make tracking more idiomatic.