improvement: show peak mem usage and instructions executed count

[gilescope]

it would be really cool to be able to get some feedback on whether changing some code had an effect on mem usage or instructions executed. I am guessing the back end of playground is using linux. The next version of criterion is going to monitor instruction counts so it's possible we could do this...

[gilescope]

Hmm, not sure what went on there. I didn't intend to add a wontfix label!