Adds a quick perf report feature to `goalc` that lets you compare how
much faster / slower it takes to compile the projects, with some simple
features like filtering the files, adjusting for how large of a margin
of error in the speeds you care about, and which test iteration you want
to compare against.
This is something I plan to use as I work more in `goalc` as an easy way
to track / show the results.
