Potential swap to long int chrono durations

[bencopl]

As @mo-mglover suggested, using long int's for the chrono durations may help avoid floating point accumulation related problems. I say "potential swap" because I don't think it's currently clear what the pros and cons of making this change will be, but those can be discussed here. An associated branch will also be made where any relevant testing be done (e.g does it negatively or positively effect the runtime and reported walltimes?).