Machine-readable output for profiling
We'll control whether or not machine-readable output is used using the
--machine-readable flag currently used for GC stats.
The format should also be self-explanatory for cost-center stats: per cost center we output the a CSV of
cc->mem_alloc. I'm a little less sure how to format the hierarchical cost center stacks; we could print out the entire trace of the stack, or maybe even organize the data recursively from the get go (outputting something JSON like, in that case.)